time: Improve C99 compliance

Quote from Linux Programmer's Manual:
  "If t is non-NULL, the return value is also stored in the memory
   pointed to by t."

Change-Id: I8cb66b67e5f34c536ce2f0db76a6dc337c42ea3f
Signed-off-by: Jim Huang <jserv@0xlab.org>
diff --git a/libc/unistd/time.c b/libc/unistd/time.c
index 13d7366..7b450c7 100644
--- a/libc/unistd/time.c
+++ b/libc/unistd/time.c
@@ -34,12 +34,15 @@
 time(time_t *t)
 {
 	struct timeval tt;
+	time_t ret;
 
 	if (gettimeofday(&tt, (struct timezone *)0) < 0)
-		return (-1);
-	if (t)
-		*t = (time_t)tt.tv_sec;
-	return (tt.tv_sec);
+		ret = -1;
+	else
+		ret = tt.tv_sec;
+	if (t != NULL)
+		*t = ret;
+	return ret;
 }