Add check to see we got a Mapped view of the file before using pointer

diff -urN objects/enhmetafile.c
--- objects/enhmetafile.c
+++ objects/enhmetafile.c
@@ -98,6 +98,7 @@
     hMapping = CreateFileMappingA( hFile, NULL, PAGE_READONLY, 0, 0, NULL );
     emh = MapViewOfFile( hMapping, FILE_MAP_READ, 0, 0, 0 );
 
+    if (!emh) return 0;
     if (emh->iType != EMR_HEADER || emh->dSignature != ENHMETA_SIGNATURE) {
         WARN("Invalid emf header type 0x%08lx sig 0x%08lx.\n",
 	     emh->iType, emh->dSignature);