Win32: Cleanup pointer test in win32_window.c

MSVC 2019 complains that the code at line 1744
(`GetMonitorInfo(window->monitor->win32.handle, &mi);`) can potentially
dereference a null pointer. The compiler is wrong in this case (it has
not spotted that `monitor` and `window->monitor` must be equal), but
I think it makes sense for our non-NULL test to be on the variable we
actually use rather than the one it was set from.

Related to #1491.

src/win32_window.c

@@ -1725,7 +1725,7 @@ void _glfwPlatformSetWindowMonitor(_GLFWwindow* window,
 
     _glfwInputWindowMonitor(window, monitor);
 
-    if (monitor)
+    if (window->monitor)
     {
         MONITORINFO mi = { sizeof(mi) };
         UINT flags = SWP_SHOWWINDOW | SWP_NOACTIVATE | SWP_NOCOPYBITS;