Fix potential violation of invariant

The shared function for reading back context attributes,
_glfwRefreshContextAttribs, did not check whether the newly created
context had been successfully made current.  This violated the invariant
that the context TLS slot always points to a valid window and that its
context is current whenever _glfw.platform.getProcAddress is called.

Related to #2327

src/context.c

@@ -363,6 +363,8 @@ GLFWbool _glfwRefreshContextAttribs(_GLFWwindow* window,
 
     previous = _glfwPlatformGetTls(&_glfw.contextSlot);
     glfwMakeContextCurrent((GLFWwindow*) window);
+    if (_glfwPlatformGetTls(&_glfw.contextSlot) != window)
+        return GLFW_FALSE;
 
     window->context.GetIntegerv = (PFNGLGETINTEGERVPROC)
         window->context.getProcAddress("glGetIntegerv");