Fix missing assertions for native access functions

src/osmesa_context.c

@@ -370,6 +370,8 @@ GLFWAPI int glfwGetOSMesaDepthBuffer(GLFWwindow* handle,
 GLFWAPI OSMesaContext glfwGetOSMesaContext(GLFWwindow* handle)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
 
     if (window->context.source != GLFW_OSMESA_CONTEXT_API)