xgettext, msgmerge: Avoid undefined non-null argument behavior

xgettext, msgmerge: Avoid undefined non-null argument behavior

* xgettext.c (remember_a_message): Building with gcc's
-fsanitize=undefined and running tests triggered:
  xgettext.c:2425:17: runtime error: null pointer passed as argument \
    2, which is declared to never be null
Adjust the argument of strncmp to never be null.
* msgmerge.c (message_merge): Likewise for memcpy.