xdisp.c 1.794 breaks Mac OS X (Carbon)

[Steven Tamm]

Just updated to latest source and a change in to xdisp.c:row_containing_pos 
causes the osx build to break reliably after scrolling through a long document. 
 Reverting to 1.793 fixes everything

The problem is that row is NULL for set_cursor_from_row in try_window_id.  
Should there be a check for row=NULL in try_window_id?  Here is the stack trace.

Exception:  EXC_BAD_ACCESS (0x0001)
Codes:      KERN_PROTECTION_FAILURE (0x0002) at 0x0000006c
Thread 0 Crashed:
 #0   0x00020488 in set_cursor_from_row (xdisp.c:9447)
 #1   0x00025d5c in try_window_id (xdisp.c:11774)
 #2   0x000230e4 in redisplay_window (xdisp.c:10478)
 #3   0x000203f8 in redisplay_window_0 (xdisp.c:9417)
 #4   0x000c9518 in internal_condition_case_1 (eval.c:1393)

Let me know if more details are needed...

-Steven