gdiplus: Fix a memory leak.

Found by Valgrind.

dlls/gdiplus/graphics.c

@@ -4161,6 +4161,7 @@ GpStatus WINGDIPAPI GdipGetClip(GpGraphics *graphics, GpRegion *region)
     /* free everything except root node and header */
     delete_element(&region->node);
     memcpy(region, clip, sizeof(GpRegion));
+    GdipFree(clip);
 
     return Ok;
 }