Fixed bug in port-share that could cause port share process to crash

Fixed bug in port-share that could cause port share process to crash
with output like this:

TCP connection established with 85.190.0.3:41781
85.190.0.3:41781 SIGTERM[soft,port-share-redirect] received, client-instance exiting
MANAGEMENT: TCP recv error: Socket operation on non-socket
MANAGEMENT: Client disconnected
MANAGEMENT: Triggering management exit
Exiting due to fatal error
EVENT: epoll_ctl EPOLL_CTL_MOD failed, sd=6: Bad file descriptor (errno=9)

Then an error like this for every incoming connection that should be
proxied:

76.120.71.74:55302 PORT SHARE: sendmsg failed -- unable to communicate with background process (6,8,-1,-1): Connection refused (errno=111)

Version 2.1.3s
(cherry picked from commit 9ed122efe870288ea75ee62a4eae2373a655145b)