Advance the advertised window (rcv_adv) to the currently received data
(rcv_nxt) if we advertising a zero window. This can be true when ACK'ing
a window probe whose one byte payload was accepted rather than dropped
because the socket's receive buffer was not completely full, but the
remaining space was smaller than the window scale.

This ensures that window probe ACKs satisfy the assumption made in r221346
and closes a window where rcv_nxt could be greater than rcv_adv.

Tested by: trasz, pho, trociny
Reviewed by: silby
MFC after: 1 week