tcp_address: check address length before manipulating it

src/tcp_address.cpp

@@ -386,7 +386,7 @@ int zmq::tcp_address_t::resolve (const char *name_, bool local_, bool ipv4only_)
     std::string port_str (delimiter + 1);
 
     //  Remove square brackets around the address, if any.
-    if (!addr_str.empty () && addr_str [0] == '[' &&
+    if (addr_str.size () >= 2 && addr_str [0] == '[' &&
           addr_str [addr_str.size () - 1] == ']')
         addr_str = addr_str.substr (1, addr_str.size () - 2);