Fabrice Bellet authored Jun 09, 2019 and Olivier Crête committed Jul 04, 2019

conn_check_prune_socket() on nsocket must be called before removing the
candidate with this socket inside the loop, to prevent the
use-after-free reported initially in issue #73.

But commit 541801d4 introduced a regression during discovery when an udp
turn over tcp socket is immediately closed by a HUP condition for
example. In this case, discovery_prune_socket() is never called, because
we don't have a candidate with this socket inside the loop. So the
nsocket is freed by the final nice_component_detach_socket() but is
still used by the discovery timer callback.

This commit moves the discovery_prune_socket() and
conn_check_prune_socket() actions before the loop instead of after, or
inside.

Closes #73