don't trust mktoken to allocate memory

Reference: /n/atom/patch/applied/token-checknil
Date: Sun Apr 19 18:07:53 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/upas/filterkit/token.c	Sun Apr 19 18:07:35 2015
+++ /sys/src/cmd/upas/filterkit/token.c	Sun Apr 19 18:07:36 2015
@@ -27,7 +27,7 @@
 check_token(char *key, char *file)
 {
 	char *s, buf[1024];
-	int i, fd;
+	int i, fd, m;
 	long now;
 
 	fd = open(file, OREAD);
@@ -41,11 +41,10 @@
 	now = time(0);
 	for(i = 0; i < 14; i++){
 		s = mktoken(key, now-24*60*60*i);
-		if(strstr(buf, s) != nil){
-			free(s);
-			return nil;
-		}
+		m = s != nil && strstr(buf, s) != nil;
 		free(s);
+		if(m)
+			return nil;
 	}
 	return "no match";
 }