Prevent segfault for formUpdateBuffer

Bug-Debian: https://github.com/tats/w3m/issues/13

form.c

@@ -481,7 +481,7 @@ formUpdateBuffer(Anchor *a, Buffer *buf, FormItemList *form)
 		spos = a->start.pos;
 		epos = a->end.pos;
 	    }
-	    if (a->start.line != a->end.line || spos > epos)
+	    if (a->start.line != a->end.line || spos > epos || epos > l->len)
 		epos = spos;
 	    pos = form_update_line(l, &p, spos, epos, COLPOS(l, epos) - col,
 				   rows > 1,