bug#29862: list-runtime-root is broken

[Danny Milosavljevic]

/gnu/store/mwvmdfksm9iwj1symfiinlikiz56s2nl-guix-0.13.0-9.ff23b47/libexec/guix/list-runtime-roots
;;; note: auto-compilation is enabled, set GUILE_AUTO_COMPILE=0
;;;       or pass the --no-auto-compile argument to disable.
;;; compiling 
/gnu/store/mwvmdfksm9iwj1symfiinlikiz56s2nl-guix-0.13.0-9.ff23b47/libexec/guix/list-runtime-roots
;;; compiled 
/home/dannym/.cache/guile/ccache/2.2-LE-8-3.A/gnu/store/mwvmdfksm9iwj1symfiinlikiz56s2nl-guix-0.13.0-9.ff23b47/libexec/guix/list-runtime-roots.go
Backtrace:
           8 (apply-smob/1 #<catch-closure c8d780>)
In ice-9/boot-9.scm:
    713:2  7 (call-with-prompt _ _ #<procedure default-prompt-handle…>)
In ice-9/eval.scm:
    619:8  6 (_ #(#(#<directory (guile-user) d10140>)))
In ice-9/boot-9.scm:
   2316:4  5 (save-module-excursion _)
  3823:12  4 (_)
In 
/gnu/store/mwvmdfksm9iwj1symfiinlikiz56s2nl-guix-0.13.0-9.ff23b47/libexec/guix/list-runtime-roots:
   145:11  3 (_)
In srfi/srfi-1.scm:
   697:23  2 (filter-map #<procedure 12ec540 at /gnu/store/mwvmdf…> . #)
In 
/gnu/store/mwvmdfksm9iwj1symfiinlikiz56s2nl-guix-0.13.0-9.ff23b47/libexec/guix/list-runtime-roots:
   138:38  1 (_ "/gnu/store")
In unknown file:
           0 (string-drop "/gnu/store" 11)

ERROR: In procedure string-drop:
ERROR: Value out of range 0 to 10: 11