mibltblt: free prgnSrcClip on error path.

Pointed out by coverity.

Signed-off-by: Dave Airlie <airlied@redhat.com>

mi/mibitblt.c

@@ -141,6 +141,8 @@ miCopyArea(DrawablePtr pSrcDrawable,
         free(ordering);
         free(pwidthFirst);
         free(pptFirst);
+        if (realSrcClip)
+            RegionDestroy(prgnSrcClip);
         return NULL;
     }