BUG/MEDIUM: servers: Add srv_addr default placeholder to the state file

When creating a state file using "show servers state" an empty field is
created in the srv_addr column if the server is from the socket family
AF_UNIX.  This leads to a warning on start up when using
"load-server-state-from-file". This patch defaults srv_addr to "-" if
the socket family is not covered.

This patch should be backported to 1.8.
diff --git a/src/proxy.c b/src/proxy.c
index 31253f1..6f71b4b 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -1450,6 +1450,9 @@
 				inet_ntop(srv->addr.ss_family, &((struct sockaddr_in6 *)&srv->addr)->sin6_addr,
 					  srv_addr, INET6_ADDRSTRLEN + 1);
 				break;
+			default:
+				memcpy(srv_addr, "-\0", 2);
+				break;
 		}
 		srv_time_since_last_change = now.tv_sec - srv->last_change;
 		bk_f_forced_id = px->options & PR_O_FORCED_ID ? 1 : 0;