fix crash on "proxysql restart"

MySQL_HostGroups_Manager::get_MyConn_from_pool() inserts connection
to mysrvc->ConnectionUsed, and this connections is also inserted
to My_Thread->cached_connections which leads to double deallocation.

Fixed https://github.com/sysown/proxysql/issues/1985.

lib/MySQL_Thread.cpp

@@ -2714,10 +2714,7 @@ MySQL_Thread::~MySQL_Thread() {
 #endif // IDLE_THREADS
 
 	if (cached_connections) {
-		while(cached_connections->len) {
-			MySQL_Connection *c=(MySQL_Connection *)cached_connections->remove_index_fast(0);
-				delete c;
-			}
+		return_local_connections();
 		delete cached_connections;
 	}