Fix missing checks for platform mismatch

The native access functions for monitor objects did not check whether
the correct platform was initialized and would return invalid handles if
it was not.

src/cocoa_monitor.m

@@ -622,6 +622,13 @@ GLFWAPI CGDirectDisplayID glfwGetCocoaMonitor(GLFWmonitor* handle)
 {
     _GLFWmonitor* monitor = (_GLFWmonitor*) handle;
     _GLFW_REQUIRE_INIT_OR_RETURN(kCGNullDirectDisplay);
+
+    if (_glfw.platform.platformID != GLFW_PLATFORM_COCOA)
+    {
+        _glfwInputError(GLFW_PLATFORM_UNAVAILABLE, "Cocoa: Platform not initialized");
+        return kCGNullDirectDisplay;
+    }
+
     return monitor->ns.displayID;
 }