BUG/MAJOR: peers: fix a crash when stopping peers on unbound processes

BUG/MAJOR: peers: fix a crash when stopping peers on unbound processes

Pradeep Jindal reported and troubleshooted a bug causing haproxy to die
during startup on all processes not making use of a peers section. It
only happens with nbproc > 1 when peers are declared. Technically it's
when the peers task is stopped on processes that don't use it that the
crash occurred (a task_free() called on a NULL task pointer).

This only affects peers v2 in the dev branch, no backport is needed.