A potential null-pointer-dereference bug

I have found a potential null-pointer-dereference bug. In the file storage/mroonga/vendor/groonga/lib/time.c, the function grn_timeval2tm may return NULL at line 119. The function grn_timeval2str in storage/mroonga/vendor/groonga/lib/time.c and rotate_log_file in storage/mroonga/vendor/groonga/lib/logger.c both call grn_timeval2tm at line 177 and line 180 respectively but don't check the return value before dereferencing it, which may lead to null-pointer-dereference vulnerability.

Although I found the potential bug in version 10.6.4, it still exists in current version.