BUG/MINOR: mux-h2: always reset rcvd_s when switching to a new frame

BUG/MINOR: mux-h2: always reset rcvd_s when switching to a new frame

In Patrick's trace it was visible that after a stream had been missed,
the next stream would receive a WINDOW_UPDATE with the first one's
credit added to its own. This makes sense because in case of error
h2c->rcvd_s is not reset. Given that this counter is per frame, better
reset it when starting to parse a new frame, it's easier and safer.

This must be backported as far as 1.8.