Use strrchr to ensure we split off the last colon for the service port

IPv6 addresses have colons and will produce invalid data for the getaddrinfo lookup.
Signed-off-by: Andrew Thompson <andy@fud.org.nz>

AUTHORS

@@ -2,6 +2,7 @@ Contributors
 ============
 
 Alexej Lotz <alexej.lotz@arcor.de>
+Andrew Thompson <andy@fud.org.nz>
 Asko Kauppi <askok@dnainternet.net>
 Barak Amar <barak.amar@gmail.com>
 Bernd Prager <bernd@prager.ws>

src/ip.cpp

@@ -272,7 +272,7 @@ int zmq::resolve_ip_hostname (sockaddr_storage *addr_, socklen_t *addr_len_,
     const char *hostname_)
 {
     //  Find the ':' that separates hostname name from service.
-    const char *delimiter = strchr (hostname_, ':');
+    const char *delimiter = strrchr (hostname_, ':');
     if (!delimiter) {
         errno = EINVAL;
         return -1;