topng: 64-bit fix

R=rsc
http://codereview.appspot.com/2124042
diff --git a/src/cmd/jpg/writepng.c b/src/cmd/jpg/writepng.c
index 76835e0..24a61a3 100644
--- a/src/cmd/jpg/writepng.c
+++ b/src/cmd/jpg/writepng.c
@@ -228,7 +228,7 @@
 	/* image chunks */
 	zr.nrow = nrow;
 	zr.ncol = ncol;
-	zr.width = rgb->width * sizeof(ulong);
+	zr.width = rgb->width * sizeof(uint32);
 	zr.data = rgb->data->bdata;
 	zr.row = zr.col = 0;
 	zr.pixwid = chantodepth(rgb->chan)/8;