BUG/MINOR: quic: idle timer task requeued in the past

BUG/MINOR: quic: idle timer task requeued in the past

When the idle timer expired with a still present mux, this task was not freed
and even requeued with a timer in the past.

Fix this issue calling task_destroy() in this case. As the task is freed,
its handler must return NULL setting local <t> variable to NULL in every cases.

Also ensure that this timer task is not armed again after having been released
with a <return> statement when this is the case from qc_idle_timer_do_rearm().

Must be backported as far as 2.6.