BUG/MINOR: quic: fix free on quic-conn fail alloc

BUG/MINOR: quic: fix free on quic-conn fail alloc

qc_new_conn() allocates several elements in intermediary steps. If one
of the fails, a global free is done on the quic_conn and its elements.
This requires that most elements are first initialized to NULL or
equivalent to ensure freeing operation is done only on proper values.

Once of this element is qc.tx.cc_buf_area. It was initialized too late
which could caused crashes. This is introduced by
  9f7cfb0a56352188854bdaef9617ca836c2a30c9
  MEDIUM: quic: Allow the quic_conn memory to be asap released.

No need to backport.