Fixed crash due to invalid pointer.

objects/metafile.c

diff --git a/objects/metafile.c b/objects/metafile.c
index 77d3fd5..6a23ac4 100644
--- a/objects/metafile.c
+++ b/objects/metafile.c
@@ -110,8 +110,8 @@
  */
 static METAHEADER *MF_GetMetaHeader( HMETAFILE hmf )
 {
-    METAFILEOBJ *metaObj = (METAFILEOBJ *)GDI_GetObjPtr( hmf, METAFILE_MAGIC );
-    return metaObj->mh;
+    METAFILEOBJ * metaObj = (METAFILEOBJ *)GDI_GetObjPtr( hmf, METAFILE_MAGIC );
+    return metaObj ? metaObj->mh : 0;
 }
 
 /******************************************************************