vertex_loader: initialize_num_total_attributes.

Keeps the public API sane.

src/video_core/vertex_loader.h

@@ -28,7 +28,7 @@ private:
     std::array<Regs::VertexAttributeFormat, 16> vertex_attribute_formats;
     std::array<u32, 16> vertex_attribute_elements{};
     std::array<bool, 16> vertex_attribute_is_default;
-    int num_total_attributes;
+    int num_total_attributes = 0;
 };
 
 }  // namespace Pica