fst_session_is_in_progress() is already checked as part of
fst_find_session_in_progress() before calling
fst_session_handle_action(). This is the only call path that can reach
fst_session_handle_tear_down() and as such, fst_session_is_in_progress()
cannot return 0 here.
Signed-off-by: Jouni Malinen <j@w1.fi>