need to account for png filter algorithm byte.  also,
since the buffer is not cleared on read, we need to insure
a full read of the header.

Reference: /n/sources/patch/applied/png-fencepost
Date: Tue Dec 25 14:39:07 CET 2012
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/jpg/readpng.c	Tue Dec 25 14:37:03 2012
+++ /sys/src/cmd/jpg/readpng.c	Tue Dec 25 14:37:02 2012
@@ -389,8 +389,8 @@
 	ZlibW zw;
 
 	buf = pngmalloc(IDATSIZE, 0);
-	Bread(b, buf, sizeof PNGmagic);
-	if(memcmp(PNGmagic, buf, sizeof PNGmagic) != 0)
+	if(Bread(b, buf, sizeof PNGmagic) != sizeof PNGmagic
+	|| memcmp(PNGmagic, buf, sizeof PNGmagic) != 0)
 		sysfatal("bad PNGmagic");
 
 	n = getchunk(b, type, buf, IDATSIZE);
--- /sys/src/cmd/jpg/writepng.c	Tue Dec 25 14:37:06 2012
+++ /sys/src/cmd/jpg/writepng.c	Tue Dec 25 14:37:04 2012
@@ -79,7 +79,7 @@
 	pixwid = z->pixwid;
 	b = buf;
 	e = b+n;
-	while(b+pixwid <= e){
+	while(b+pixwid < e){	/* one less for filter alg byte */
 		if(z->y >= z->dy)
 			break;
 		if(z->x == 0)