avoid the pitfalls of the usual conversions when widening
ulong to 64 bits.

Reference: /n/patches.lsub.org/patch/memdrawneg
Date: Wed Oct 10 16:44:39 CES 2012
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libmemdraw/addr.c	Sun Sep  9 02:53:33 2012
+++ /sys/src/libmemdraw/addr.c	Wed Oct 10 07:47:49 2012
@@ -17,7 +17,8 @@
 {
 	uchar *a;
 
-	a = i->data->bdata+i->zero+sizeof(ulong)*p.y*i->width;
+	/* careful to sign-extend negative p.y for 64-bits */
+	a = i->data->bdata+i->zero+(int)(sizeof(ulong)*p.y*i->width);
 
 	if(i->depth < 8){
 		/*