BUG/MINOR: quic: fix qc.cids access on quic-conn fail alloc

BUG/MINOR: quic: fix qc.cids access on quic-conn fail alloc

CIDs tree is now allocated dynamically since the following commit :
  276697438d50456f92487c990f20c4d726dfdb96
  MINOR: quic: Use a pool for the connection ID tree.

This can caused a crash if qc_new_conn() is interrupted due to an
intermediary failed allocation. When freeing all connection members,
free_quic_conn_cids() is used. However, this function does not support a
NULL cids.

To fix this, simply check that cids is NULL during free_quic_conn_cids()
prologue.

This bug was reproduced using -dMfail.

No need to backport.