BUG/MEDIUM: mux-quic: do not crash on qcs_destroy for connection error

BUG/MEDIUM: mux-quic: do not crash on qcs_destroy for connection error

On qcs_destroy(), a BUG_ON() statement check that QCS does not have
anymore prepared data. This is to ensure connection flow control is
always coherent and prevent transfer freeze.

However, this BUG_ON() may cause a spurrious crash in case QCC is
considered on error. Indeed, in this case, all transfers are interrupted
and qmux_strm_detach() will proceed to immediate QCS free before
releasing the connection. In this situation, connection flow control is
irrelevant so the BUG_ON() should be ignored.

This crash occurs since the MUX refactoring via the following patch.
Previously, a similar BUG_ON() was used but it was incorrectly
implemented rendering it immune even to targetted cause.

  3fe3251593e32c7ee07be94a193aea3a8eefb076
  MEDIUM: mux-quic: simplify sending API

This should fix github issue #2456.

This does not need to be backported.