I hate to say this, but I lost that gdb session. I am still able to consistently segfault on startup (when I load nlinum in after-init-hook). But this time, it's at a different point. Sorry about that.
Thread 1 "emacs" received signal SIGSEGV, Segmentation fault.
0x00000033e307a13c in _int_malloc () from /lib64/libc.so.6
(gdb) bt
#0 0x00000033e307a13c in _int_malloc () from /lib64/libc.so.6
#1 0x00000033e307ab1c in malloc () from /lib64/libc.so.6
#2 0x00000000005464ee in lmalloc (size=8188) at alloc.c:1414
#3 lisp_malloc (nbytes=<optimized out>, type=MEM_TYPE_NON_LISP) at alloc.c:1063
#4 0x0000000000547bef in allocate_string_data (s=0x4fd6600, nchars=369, nbytes=370) at alloc.c:1998
#5 0x0000000000547dc7 in make_uninit_multibyte_string (nchars=369, nbytes=370) at alloc.c:2513
#6 0x000000000056ab9b in concat (nargs=25, args=0x7fffffff1750, target_type=<optimized out>, last_special=<optimized out>) at fns.c:637
#7 0x000000000056b760 in Fmapconcat (function=60420685, sequence=<optimized out>, separator=60725716) at fns.c:2562
#8 0x000000000055fbb4 in Ffuncall (nargs=<optimized out>, args=0x7fffffff1958) at eval.c:2706
#9 0x000000000059699d in exec_byte_code (bytestr=<optimized out>, vector=60420725, maxdepth=<optimized out>,
args_template=<optimized out>, nargs=<optimized out>, args=<optimized out>) at bytecode.c:880
#10 0x000000000055f5fa in funcall_lambda (fun=60420901, nargs=<optimized out>, arg_vector=0x7fffffff1b30) at eval.c:2929
#11 0x000000000055f943 in Ffuncall (nargs=<optimized out>, args=0x7fffffff1b28) at eval.c:2760
#12 0x000000000059699d in exec_byte_code (bytestr=<optimized out>, vector=61050749, maxdepth=<optimized out>,
args_template=<optimized out>, nargs=<optimized out>, args=<optimized out>) at bytecode.c:880
#13 0x000000000055f5fa in funcall_lambda (fun=61051037, nargs=<optimized out>, arg_vector=0x7fffffff1c60) at eval.c:2929
#14 0x000000000055ebeb in apply_lambda (fun=61051037, args=0, count=13) at eval.c:2800
#15 0x000000000055eeb6 in eval_sub (form=<optimized out>) at eval.c:2247
#16 0x0000000000560c92 in Feval (form=61021091, lexical=<optimized out>) at eval.c:1994
#17 0x000000000055fbc8 in Ffuncall (nargs=<optimized out>, args=0x7fffffff1df8) at eval.c:2702
#18 0x000000000055e6ce in internal_condition_case_n (bfun=0x55f7a0 <Ffuncall>, nargs=2, args=0x7fffffff1eb0, handlers=<optimized out>,
hfun=0x447c60 <safe_eval_handler>) at eval.c:1395
#19 0x000000000043ae89 in safe__call (inhibit_quit=true, nargs=2, func=<optimized out>, ap=<optimized out>) at xdisp.c:2558
#20 0x000000000043b042 in safe__call1 (inhibit_quit=<optimized out>, fn=<optimized out>) at xdisp.c:2595
#21 0x000000000044fcc3 in safe__eval (sexpr=<optimized out>, inhibit_quit=true) at xdisp.c:2609
#22 display_mode_element (it=0x7fffffff2360, depth=4, field_width=0, precision=-82, elt=61021075, props=0, risky=false) at xdisp.c:22863
#23 0x000000000044fe8e in display_mode_element (it=0x7fffffff2360, depth=3, field_width=0, precision=-82, elt=61119507, props=0,
risky=false) at xdisp.c:22944
#24 0x000000000044fe8e in display_mode_element (it=0x7fffffff2360, depth=1, field_width=0, precision=0, elt=61156931, props=0, risky=false)
at xdisp.c:22944
#25 0x0000000000454cf9 in display_mode_line (w=0x11ec2c0, face_id=MODE_LINE_FACE_ID, format=61157123) at xdisp.c:22460
#26 0x0000000000454fee in display_mode_lines (w=0x11ec2c0) at xdisp.c:22402
#27 0x00000000004602f7 in redisplay_window (window=18793157, just_this_one_p=false) at xdisp.c:17066
#28 0x0000000000463b36 in redisplay_window_0 (window=<optimized out>) at xdisp.c:14491
#29 0x000000000055e7c6 in internal_condition_case_1 (bfun=0x463b10 <redisplay_window_0>, arg=18793157, handlers=<optimized out>,
hfun=0x429b40 <redisplay_window_error>) at eval.c:1339
#30 0x000000000044612e in redisplay_windows (window=<optimized out>) at xdisp.c:14471
#31 0x000000000045cfd5 in redisplay_internal () at xdisp.c:14031
#32 0x00000000004f5299 in read_char (commandflag=1, map=100930259, prev_event=0, used_mouse_menu=0x7fffffffb11f, end_time=0x0)
at keyboard.c:2482
#33 0x00000000004f90c0 in read_key_sequence (keybuf=0x7fffffffb190, prompt=0, dont_downcase_last=false, can_return_switch_frame=true,
fix_current_buffer=true, prevent_redisplay=false, bufsize=30) at keyboard.c:9068
#34 0x00000000004fa3ba in command_loop_1 () at keyboard.c:1370
#35 0x000000000055e82a in internal_condition_case (bfun=0x4fa1f0 <command_loop_1>, handlers=<optimized out>, hfun=0x4f8200 <cmd_error>)
at eval.c:1315
#36 0x00000000004f81ec in command_loop_2 (ignore=<optimized out>) at keyboard.c:1112
#37 0x000000000055e8b8 in internal_catch (tag=<optimized out>, func=0x4f81d0 <command_loop_2>, arg=0) at eval.c:1080
#38 0x00000000004f7f67 in command_loop () at keyboard.c:1091
#39 0x00000000004f7ff5 in recursive_edit_1 () at keyboard.c:697
#40 0x00000000004f8135 in Frecursive_edit () at keyboard.c:768
#41 0x00000000004e997e in main (argc=<optimized out>, argv=<optimized out>) at emacs.c:1629