BUG/MINOR: h3: fix return value on decode_qcs on error

BUG/MINOR: h3: fix return value on decode_qcs on error

Convert return code to -1 when an error has been detected. This is
required since the previous API change on return value from the patch :

  1f21ebdd7686bf435682cacd31e635db0c65b061
  MINOR: mux-quic/h3: adjust demuxing function return values

Without this, QUIC MUX won't consider the call as an error and will try
to remove one byte from the buffer. This may cause a BUG_ON failure if
the buffer is empty at this stage.

This bug was introduced in the current dev tree. Does not need to be
backported.