BUG/MINOR: h3: properly handle alloc failure on finalize

BUG/MINOR: h3: properly handle alloc failure on finalize

If H3 control stream Tx buffer cannot be allocated, return a proper
errur through h3_finalize(). This will cause the emission of a
CONNECTION_CLOSE with error H3_INTERNAL_ERROR and closure of the whole
connection.

This should be backported up to 2.6. Note that 2.9 has some difference
which will cause conflict. The main one is that qcc_get_stream_txbuf()
does not exist in this version. Instead the check in h3_control_send()
should be made after mux_get_buf(). Finally, it may be useful to first
pick previous commit (MINOR: h3: add traces for connection init stage)
to improve context similarity.