live555 : check for NULL string in OPTION callback

modules/demux/live555.cpp

@@ -550,7 +550,7 @@ static void continueAfterOPTIONS( RTSPClient* client, int result_code,
     }
     else
     {
-        p_sys->b_get_param = (bool)strstr( result_string, "GET_PARAMETER" );
+        p_sys->b_get_param = result_string != NULL && strstr( result_string, "GET_PARAMETER" ) != NULL;
         client->sendDescribeCommand( continueAfterDESCRIBE );
     }
     delete[] result_string;