Win32: Add missing out parameter NULL checks

This fixes _glfwPlatformGetMonitorWorkarea not checking if its out
parameters were NULL.

Related to #1322.

src/win32_monitor.c

@@ -377,9 +377,13 @@ void _glfwPlatformGetMonitorWorkarea(_GLFWmonitor* monitor, int* xpos, int* ypos
     hMonitor = MonitorFromPoint( pointInMonitor, 0 );
     GetMonitorInfo(hMonitor, &monitorInfo);
 
+    if (xpos)
         *xpos   = monitorInfo.rcWork.left;
+    if (ypos)
         *ypos   = monitorInfo.rcWork.top;
+    if (width)
         *width  = monitorInfo.rcWork.right - monitorInfo.rcWork.left;
+    if (height)
         *height = monitorInfo.rcWork.bottom - monitorInfo.rcWork.top;
 }