BUG/MINOR: checks: Set missing id to the dummy checks frontend

BUG/MINOR: checks: Set missing id to the dummy checks frontend

The dummy frontend used to create the session of the tcp-checks is
initialized without identifier. However, it is required because this id may
be used without any guard, for instance in log-format string via "%f" or
when fe_name sample fetch is called. Thus, an unset id may lead to crashes.

This patch must be backported as far as 2.2.

(cherry picked from commit 0f1fc23d4e3060c4277c11c9f1f41c27d8e54fbf)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>