tcp_address: make port number conversion more robust

This still rejects 00 as port number.

src/tcp_address.cpp

@@ -392,10 +392,10 @@ int zmq::tcp_address_t::resolve (const char *name_, bool local_, bool ipv4only_)
 
     uint16_t port;
     // Allow 0 specifically, to detect invalid port error in atoi if not
-    if (port_str[0] == '*' || port_str[0] == '0') {
+    if (port_str == "*" || port_str == "0")
         // Resolve wildcard to 0 to allow autoselection of port
         port = 0;
-    } else {
+    else {
         //  Parse the port number (0 is not a valid port).
         port = (uint16_t) atoi (port_str.c_str());
         if (port == 0) {