Place assertions for handles after init check

This lets automated testing check that GLFW_NOT_INITIALIZED is emitted
for every public function.

src/nsgl_context.m

@@ -362,9 +362,6 @@ GLFWbool _glfwCreateContextNSGL(_GLFWwindow* window,
 
 GLFWAPI id glfwGetNSGLContext(GLFWwindow* handle)
 {
-    _GLFWwindow* window = (_GLFWwindow*) handle;
-    assert(window != NULL);
-
     _GLFW_REQUIRE_INIT_OR_RETURN(nil);
 
     if (_glfw.platform.platformID != GLFW_PLATFORM_COCOA)
@@ -374,6 +371,9 @@ GLFWAPI id glfwGetNSGLContext(GLFWwindow* handle)
         return nil;
     }
 
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+
     if (window->context.source != GLFW_NATIVE_CONTEXT_API)
     {
         _glfwInputError(GLFW_NO_WINDOW_CONTEXT, NULL);