BUG/MINOR: mux-quic: fix crash on aborting uni remote stream

BUG/MINOR: mux-quic: fix crash on aborting uni remote stream

A remote unidirectional stream can be aborted prematurely if application
layers cannot identify its type. In this case, a STOP_SENDING frame is
emitted.

Since QUIC MUX refactoring, a crash would occur in this scenario due to
2 specific characteristics of remote uni streams :
* qcs.tx.fctl was not initialized completely. This cause a crash due to
  BUG_ON() statement inside qcs_destroy().
* qcs.stream is never allocated. This caused qcs_prep_bytes() to crash
  inside qcc_io_send().

This bug is considered minor as it happens only on very specific QUIC
clients. It was detected when using s2n-quic over interop.

This does not need to be backported.