xlibi18n/lcFile.c: avoid use of possibly-NULL pointer with strcpy
Fixes gcc warnings:
lcFile.c: In function ‘_XlcLocaleLibDirName’:
lcFile.c:708:5: warning: use of possibly-NULL ‘last_dir_name’ where
non-null expected [CWE-690] [-Wanalyzer-possible-null-argument]
708 | strcpy (last_dir_name, dir_name);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~