bridge_softmix.c: Fix memory leak.

Made release the media_types vector in
softmix_bridge_stream_topology_changed().

Change-Id: Ide3f47e379b614879220dfeeb13843f9f2b177b5

bridges/bridge_softmix.c

@@ -2218,6 +2218,8 @@ static void softmix_bridge_stream_topology_changed(struct ast_bridge *bridge, st
 		ast_channel_unlock(participant->chan);
 		ast_bridge_channel_unlock(participant);
 	}
+
+	AST_VECTOR_FREE(&media_types);
 }
 
 static struct ast_bridge_technology softmix_bridge = {