Call va_end on ap when vsnprintf fails in run_cmd(..) to clean up

the variable state

MFC after: 1 week
Reported by: cppcheck
Sponsored by: EMC / Isilon Storage Division

sbin/swapon/swapon.c

@@ -642,6 +642,7 @@ run_cmd(int *ofd, const char *cmdline, ...)
 	rv = vasprintf(&cmd, cmdline, ap);
 	if (rv == -1) {
 		warn("%s", __func__);
+		va_end(ap);
 		return (rv);
 	}
 	va_end(ap);