BUG/MINOR: quic: prevent crash on qc_kill_conn()

BUG/MINOR: quic: prevent crash on qc_kill_conn()

Ensure idle_timer task is allocated in qc_kill_conn() before waking it
up. It can be NULL if idle timer has already fired but MUX layer is
still present, which prevents immediate quic_conn release.

qc_kill_conn() is only used on send() syscall fatal error to notify
upper layer of an error and close the whole connection asap.

This crash occurence is pretty rare as it relies on timing issues. It
happens only if idle timer occurs before the MUX release (a bigger
client timeout is thus required) and any send() syscall detected error.
For now, it was only reproduced using GDB to interrupt haproxy longer
than the idle timeout.

This should be backported up to 2.6.