Clean-up: sessiond: cmd: fix bogus -Wmaybe-uninitialized

Old gcc versions report:
cmd.c:4751: ‘query_target_action’ may be used uninitialized in this function [-Wmaybe-uninitialized]

For this to be reachable, a query target would have to change suddenly
during the execution of the function.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id90ca37e232385f73d54558f2a00d01cbf773a92