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kfs: fix read offset integer overflow

--- a/sys/src/cmd/disk/kfs/9p1.c

+++ b/sys/src/cmd/disk/kfs/9p1.c

@@ -807,7 +807,9 @@

 		addr = 0;

 		goto dread;

 	}

-	if(offset+count > d->size)

+	if(offset >= d->size)

+		count = 0;

+	else if(offset+count > d->size)

 		count = d->size - offset;

 	while(count > 0) {

 		addr = offset / BUFSIZE;

--- a/sys/src/cmd/disk/kfs/9p2.c

+++ b/sys/src/cmd/disk/kfs/9p2.c

@@ -1071,7 +1071,9 @@

 	accessdir(p, d, FREAD);

 	if(d->mode & DDIR)

 		goto dread;

-	if(offset+count > d->size)

+	if(offset >= d->size)

+		count = 0;

+	else if(offset+count > d->size)

 		count = d->size - offset;

 	while(count > 0){

 		if(p == nil){