libgrep: Don't crash upon out-of-memory.
Pinpointed by gcc 13 warning:
warning: use of possibly-NULL ‘mb_properties’ where non-null expected [CWE-690] [-Wanalyzer-possible-null-argument]
* gettext-tools/libgrep/m-fgrep.c (check_multibyte_string): Check the malloc()
return value.
