nanocoap_sock: fix wrong assertion
nanocoap_sock: fix wrong assertion

The assertion is a bit overeager.
In case of receiving a wrong message ID, we re-try receive without
entering the STATE_REQUEST_SEND state again, so it is expected that
we get a non-NULL ctx/response from sock_udp_recv_buf().

What this assert should actually check is that we don't get a non-NULL
ctx after calling sock_udp_recv_buf() with a non-NULL ctx.

So make this explicit to not falsely fail the assertion.

RIOT-OS/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT,kaspar030/RIOT,kYc0o/RIOT