Events: fixed handling zero-length client address.

Events: fixed handling zero-length client address.

On Linux recvmsg() syscall may return a zero-length client address when
receiving a datagram from an unbound unix datagram socket.  It is usually
assumed that socket address has at least the sa_family member.  Zero-length
socket address caused buffer over-read in functions which receive socket
address, for example ngx_sock_ntop().  Typically the over-read resulted in
unexpected socket family followed by session close.  Now a fake socket address
is allocated instead of a zero-length client address.