OpenGL: Don't attempt to draw empty triangle batches

Our code did not handle this well, causing random crashes in some
situations.

src/video_core/renderer_opengl/gl_rasterizer.cpp

@@ -190,6 +190,9 @@ void RasterizerOpenGL::AddTriangle(const Pica::Shader::OutputVertex& v0,
 }
 
 void RasterizerOpenGL::DrawTriangles() {
+    if (vertex_batch.empty())
+        return;
+
     SyncFramebuffer();
     SyncDrawState();