kernel: mem_slab: always validate memory address on free

Allowing an invalid address to be "freed" when asserts are disabled
is dangerous and can lead to a very hard class of bugs (and potential
security issues) to troubleshoot. This change always validates the
address before adding it to the free list and calls k_panic() if
asserts are not enabled.

Signed-off-by: Corey Wharton <xodus7@cwharton.com>