remove circular shared_ptr and a few other memory leaks

src/mbgl/map/vector_tile_data.cpp

@@ -36,6 +36,10 @@ void VectorTileData::parse() {
     }
 
     try {
+        if (!style) {
+            throw std::runtime_error("style isn't present in VectorTileData object anymore");
+        }
+
         // Parsing creates state that is encapsulated in TileParser. While parsing,
         // the TileParser object writes results into this objects. All other state
         // is going to be discarded afterwards.
@@ -43,6 +47,9 @@ void VectorTileData::parse() {
                           glyphAtlas, glyphStore,
                           spriteAtlas, sprite,
                           texturePool);
+        // Clear the style so that we don't have a cycle in the shared_ptr references.
+        style.reset();
+
         parser.parse();
     } catch (const std::exception& ex) {
         Log::Error(Event::ParseTile, "Parsing [%d/%d/%d] failed: %s", id.z, id.x, id.y, ex.what());