wined3d: Set the currently active context to NULL when it's destroyed.

This should make us crash when trying to use the "currently active" context
after it has been destroyed, rather than messing around with freed memory.

dlls/wined3d/context.c

@@ -1010,6 +1010,12 @@ void DestroyContext(IWineD3DDeviceImpl *This, WineD3DContext *context) {
 
     LEAVE_GL();
 
+    if (This->activeContext == context)
+    {
+        This->activeContext = NULL;
+        TRACE("Destroying the active context.\n");
+    }
+
     /* Cleanup the GL context */
     pwglMakeCurrent(NULL, NULL);
     if(context->isPBuffer) {