free getenv memory

Reference: /n/atom/patch/applied/samtermgetenvfree
Date: Sat May  3 06:06:27 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/samterm/plan9.c	Sat May  3 06:06:21 2014
+++ /sys/src/cmd/samterm/plan9.c	Sat May  3 06:06:21 2014
@@ -38,6 +38,7 @@
 	t = getenv("tabstop");
 	if(t != nil)
 		maxtab = strtoul(t, nil, 0);
+	free(t);
 	draw(screen, screen->clipr, display->white, nil, ZP);
 }