posix: timers: correct pointer passed to k_mem_slab_free()

If it is not possible to create a timer in timer_create(),
then the timer_obj associated with the timer must be freed.

However, the address of the pointer was mistakenly being
passed to k_mem_slab_free() rather than simply the
the pointer.

This caused a crash in tests which can easily be avoided
by passing the pointer rather than the address of the
pointer.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>