Fixed typo ensuring header str is \0 terminated

This was obviously intended to send an empty header if
php_gmtime_r() fails. Currently it could send garbage.

Author: weltling

ext/session/session.c

@@ -1052,7 +1052,7 @@ static inline void strcpy_gmt(char *ubuf, time_t *when) /* {{{ */
 	res = php_gmtime_r(when, &tm);
 
 	if (!res) {
-		buf[0] = '\0';
+		ubuf[0] = '\0';
 		return;
 	}