kernel: mem_slab: only define slab_ptr_is_good with assert enabled

Add a __ASSERT_ON guard around slab_ptr_is_good, as that is only used in
assertions and leaving it on seems to generate a build warning with some
clang versions:

kernel/mem_slab.c:207:20: error: unused function 'slab_ptr_is_good'
  207 | static inline bool slab_ptr_is_good(struct k_mem_slab *slab,...
      |                    ^~~~~~~~~~~~~~~~

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>