MDEV-304: Insufficient buffer allocation for Query_log_event

The constructor for Query_log_event allocated 2 bytes too few for
extra space needed by Query cache. (Not sure if this is reproducible
in practice, as there are often a couple of extra bytes allocated
for unused string zero terminators, but better safe than sorry).

sql/log_event.cc

@@ -3147,6 +3147,7 @@ Query_log_event::Query_log_event(const char* buf, uint event_len,
   if (!(start= data_buf = (Log_event::Byte*) my_malloc(catalog_len + 1 +
                                               time_zone_len + 1 +
                                               data_len + 1 +
+                                              QUERY_CACHE_DB_LENGTH_SIZE +
                                               QUERY_CACHE_FLAGS_SIZE +
                                               user.length + 1 +
                                               host.length + 1 +