auditsc: audit_log_single_execve_arg() returns signed

"len" needs to be signed because audit_log_single_execve_arg() returns -1 and we want to check for that:

                len = audit_log_single_execve_arg(context, ab, i,
                                                  &len_sent, p, buf);
                if (len <= 0)
                        break;

index 3828ad5..2c060c3 100644
@@ -1158,7 +1158,8 @@ static void audit_log_execve_info(struct audit_context *context,
 				  struct audit_aux_data_execve *axi)
 {
 	int i;
-	size_t len, len_sent = 0;
+	ssize_t len;
+	size_t len_sent = 0;
 	const char __user *p;
 	char *buf;