Initialize server->serial in all cases

Running scrcpy --tcpip on a device already connected via TCP/IP did not
initialize server->serial.

As a consequence, in debug mode, an assertion failed:

    scrcpy: ../app/src/server.c:770: run_server: Assertion
    `server->serial' failed.

In release mode, scrcpy failed with this error:

    adb: -s requires an argument

app/src/server.c

@@ -689,6 +689,11 @@ sc_server_configure_tcpip_unknown_address(struct sc_server *server,
     if (is_already_tcpip) {
         // Nothing to do
         LOGI("Device already connected via TCP/IP: %s", serial);
+        server->serial = strdup(serial);
+        if (!server->serial) {
+            LOG_OOM();
+            return false;
+        }
         return true;
     }