Properly detect MMDB lookup failures

Properly detect MMDB lookup failures

Only comparing the value of the integer passed as the last argument to
MMDB_lookup_sockaddr() against MMDB_SUCCESS is not enough to ensure that
an MMDB lookup was successful - the 'found_entry' field of the
MMDB_lookup_result_s structure returned by that function also needs to
be true or else the remaining contents of that structure should be
ignored as the lookup failed.  Extend the relevant logical condition in
get_entry_for() to ensure the latter does not return incorrect MMDB
entries for IP addresses which do not belong to any subnet defined in a
given GeoIP2 database.

(cherry picked from commit ec8334fb743ecd8ae29f2751dab0fd86b7334327)