BUG/MEDIUM: quic-be: avoid crashes when releasing Initial pktns

BUG/MEDIUM: quic-be: avoid crashes when releasing Initial pktns

This bug arrived with this fix:

    BUG/MINOR: quic-be: missing Initial packet number space discarding

leading to crashes when dereferencing ->ipktns.

Such crashes could be reproduced with -dMfail option. To reach them, the
memory allocations must fail. So, this is relatively rare, except on systems
with limited memory.

To fix this, do not call quic_pktns_discard() if ->ipktns is NULL.

No need to backport.