libobs-d3d11: Only load vertex buffer before drawing

Fixes a bug where loading vertex shaders could cause error messages
about mismatching vertex buffer data to appear because the vertex shader
would try to reload the previously used vertex buffer.

libobs-d3d11/d3d11-shader.cpp

@@ -287,6 +287,8 @@ void gs_shader::UploadParams()
 
 void gs_shader_destroy(gs_shader_t *shader)
 {
+	if (shader && shader->device->lastVertexShader == shader)
+		shader->device->lastVertexShader = nullptr;
 	delete shader;
 }