gdiplus: Fix a memory leak.

Found by Valgrind.
diff --git a/dlls/gdiplus/graphics.c b/dlls/gdiplus/graphics.c
index 80ef463..b1092b7 100644
--- a/dlls/gdiplus/graphics.c
+++ b/dlls/gdiplus/graphics.c
@@ -4161,6 +4161,7 @@
     /* free everything except root node and header */
     delete_element(&region->node);
     memcpy(region, clip, sizeof(GpRegion));
+    GdipFree(clip);
 
     return Ok;
 }