BUG/MINOR: h3: close connection on sending alloc errors

BUG/MINOR: h3: close connection on sending alloc errors

When encoding new HTTP/3 frames, QCS Tx buffer must be allocated if
currently NULL. Previously, allocation failure was not properly checked,
leaving the connection in an unspecified state, or worse risking a
crash.

Fix this by setting <h3c.err> to H3_INTERNAL_ERROR each time the
allocation fails. This will stop sending and close the connection. In
the future, it may be better to put the connection on pause waiting for
allocation to succeed but this is too complicated to implement for now
in a reliable way.

Along with the current change, return of all HTX parsing functions
(h3_resp_*_send) were set to a negative value in case of error. A new
BUG_ON() in h3_snd_buf() ensures that if such a value is returned,
either a connection error is register (via <h3c.err>) or buffer is
temporarily full (flag QC_SF_BLK_MROOM).

This should fix github issue #2389.

This should be backported up to 2.6. Note that qcc_get_stream_txbuf()
does not exist in 2.9 and below. mux_get_buf() is its equivalent. An
explicit check b_is_null(&qcs.tx.buf) should be used there.