BUG/MINOR: mux-h2: use CANCEL, not STREAM_CLOSED in h2c_frt_handle_data()

BUG/MINOR: mux-h2: use CANCEL, not STREAM_CLOSED in h2c_frt_handle_data()

There is a test on the existence of the conn_stream when receiving data,
to be sure to have somewhere to deliver it. Right now it responds with
STREAM_CLOSED, which is not correct since from an H2 point of view the
stream is not closed and a peer could be upset to see this. After some
analysis, it is important to keep this test to be sure not to fill the
rxbuf then stall the connection. Another option could be to modiffy
h2_frt_transfer_data() to silently discard any contents but the CANCEL
error code is designed exactly for this and to save the peer from
continuing to stream data that will be discarded, so better switch to
using this.

This must be backported as far as 1.8.