acme: fix Get of dir in nameless window (thanks Colton Lewis)

TBR=r
https://codereview.appspot.com/89390043

src/cmd/acme/text.c

diff --git a/src/cmd/acme/text.c b/src/cmd/acme/text.c
index c055c5e..7ea6de9 100644
--- a/src/cmd/acme/text.c
+++ b/src/cmd/acme/text.c
@@ -228,7 +228,7 @@
 		}
 		t->w->isdir = TRUE;
 		t->w->filemenu = FALSE;
-		if(t->file->name[t->file->nname-1] != '/'){
+		if(t->file->nname > 0 && t->file->name[t->file->nname-1] != '/'){
 			rp = runemalloc(t->file->nname+1);
 			runemove(rp, t->file->name, t->file->nname);
 			rp[t->file->nname] = '/';