dns-proxy: Don't use proxy socket if we fail to bypass it

This will result in an infinite loop as packets sent over that socket
will again pass through the TUN device and the DNS proxy.

Apparently, bypassing fails when airplane mode is enabled.

Fixes #662.

src/frontends/android/jni/libandroidbridge/backend/android_dns_proxy.c

@@ -125,6 +125,8 @@ static int open_socket(int family)
 												skt, family))
 	{
 		DBG1(DBG_NET, "installing bypass policy for proxy socket failed");
+		close(skt);
+		return -1;
 	}
 	return skt;
 }