Problem: zmq_connect doesn't return EINVAL on invalid endpoint

Solution: set errno to EINVAL when tcp:// endpoint is invalid (was just
leaving errno to previous value).

src/socket_base.cpp

@@ -619,6 +619,7 @@ int zmq::socket_base_t::connect (const char *addr_)
             }
         }
         if (rc == -1) {
+            errno = EINVAL;
             delete paddr;
             return -1;
         }