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samterm: avoid division-by-zero when $tabstop is zero or empty

this happens in libframe:

	/sys/src/libframe/frutil.c:80: 	x -= (x-f->r.min.x)%f->maxtab;

but there's no way to control when the user changes the
maxtab value, so it's samterm's responsibility to
sanitize it.

--- a/sys/src/cmd/samterm/plan9.c

+++ b/sys/src/cmd/samterm/plan9.c

@@ -26,6 +26,7 @@

 getscreen(int argc, char **argv)

 {

 	char *t;

+	uint tlen;

 	ARGBEGIN{

 	case 'a':

@@ -43,8 +44,8 @@

 		threadexitsall("init");

 	}

 	t = getenv("tabstop");

-	if(t != nil)

-		maxtab = strtoul(t, nil, 0);

+	if(t != nil && (tlen = strtoul(t, nil, 0)) > 0)

+		maxtab = tlen;

 	free(t);

 	draw(screen, screen->clipr, display->white, nil, ZP);

 }

-- 

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