builtin/push: call set_refspecs after validating remote

Since 9badf97c (remote: allow resetting url list), we reset the remote URL if the provided URL is empty. This means any caller of remotes_remote_get() would now get a NULL remote.

The 'builtin/push.c' code, calls 'set_refspecs' before validating the remote. This worked earlier since we would get a remote, albeit with an empty URL. With the new changes, we get a NULL remote and this crashes.