BUG/MINOR: quic: Possible crashes in qc_idle_timer_task()

BUG/MINOR: quic: Possible crashes in qc_idle_timer_task()

This is due to this commit:

    MINOR: quic: Add trace to debug idle timer task issues

where has been added without having been tested at developer level.
<qc> was dereferenced after having been released by qc_conn_release().

Set qc to NULL value after having been released to forbid its dereferencing.
Add a check for qc->idle_timer_task in the traces added by the mentionned
commit above to prevent its dereferencing if NULL.
Take the opportunity of this patch to modify trace events from
QUIC_EV_CONN_SSLALERT to QUIC_EV_CONN_IDLE_TIMER.

Must be backported to 2.6 and 2.7.