gdiplus: Zero the MetafileHeader structure in GetMetafileHeader.

Since we return success, we should at least make sure the result is not
uninitialized memory.

dlls/gdiplus/image.c

@@ -2108,6 +2108,8 @@ GpStatus WINGDIPAPI GdipGetMetafileHeaderFromMetafile(GpMetafile * metafile,
     if(!(calls++))
         FIXME("not implemented\n");
 
+    memset(header, 0, sizeof(MetafileHeader));
+
     return Ok;
 }