Add an assertion to xdisp.c

* src/xdisp.c (display_mode_element): Add an assertion where
we assume that 'string' returned by decode_mode_spec is always
either a Lisp string or nil.
1 job for master in 50 minutes and 39 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #1696
test-all

00:50:39

 
Name Stage Failure
failed
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:957: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1