Check OOM condition in value_for_key_as_string
Check OOM condition in value_for_key_as_string

The function mmdb_strndup can return NULL if system runs out of
memory. This return value is generally checked, but not in function
value_for_key_as_string.

Return MMDB_OUT_OF_MEMORY_ERROR instead of MMDB_SUCCESS if that
happens.

maxmind/libmaxminddb,maxmind/libmaxminddb,maxmind/libmaxminddb