MDEV-12951 Server crash [mysqld got exception 0xc0000005]

Same as MDEV-12725 but for federated (not X).
Set and reset mysql.net->thd appropriately.

mysql-test/suite/federated/net_thd_crash-12951.result

+set global query_cache_size= 16*1024*1024;
+set global query_cache_type= 1;
+create table t1 (i int) engine=innodb;
+create table t2 (i int) engine=federated 
+CONNECTION="mysql://root@localhost:MASTER_MYPORT/test/t1";
+select * from t2;
+i
+drop table t2;
+drop table t1;
+set global query_cache_type= default;
+set global query_cache_size= default;

mysql-test/suite/federated/net_thd_crash-12951.test

+#
+# MDEV-12951 Server crash [mysqld got exception 0xc0000005]
+#
+
+--source include/have_innodb.inc
+ 
+set global query_cache_size= 16*1024*1024;
+set global query_cache_type= 1;
+ 
+create table t1 (i int) engine=innodb;
+--replace_result $MASTER_MYPORT MASTER_MYPORT
+eval create table t2 (i int) engine=federated 
+  CONNECTION="mysql://root@localhost:$MASTER_MYPORT/test/t1";
+ 
+select * from t2;
+
+source include/restart_mysqld.inc;
+
+drop table t2;
+drop table t1;
+
+set global query_cache_type= default;
+set global query_cache_size= default;

storage/federated/ha_federated.cc

@@ -2980,6 +2980,9 @@ int ha_federated::reset(void)
   }
   reset_dynamic(&results);
 
+  if (mysql)
+    mysql->net.thd= NULL;
+
   return 0;
 }
 
@@ -3200,12 +3203,14 @@ int ha_federated::real_query(const char *query, size_t length)
   int rc= 0;
   DBUG_ENTER("ha_federated::real_query");
 
-  if (!mysql && (rc= real_connect()))
+  if (!query || !length)
     goto end;
 
-  if (!query || !length)
+  if (!mysql && (rc= real_connect()))
     goto end;
 
+  mysql->net.thd= table->in_use;
+
   rc= mysql_real_query(mysql, query, (uint) length);
   
 end: