mempool can return success if no memory was available

[pdunaj]

Describe the bug
We replicated a bug where assertion on data failed. Problem replicates randomly and rarely (I would say once per hour of operation). Quick investigation shown that application gets 0x4 from k_malloc. Quick look at the code shown that this is possible if _sys_mem_pool_block_alloc returns success but block.data is NULL.
Looking at _sys_mem_pool_block_alloc proved that this is possible as the loop that is allocating a block gives no guarantee of actually allocating (i.e. data can be NULL) but it is always returning zero.

As a bonus there is no lock taken when level_empty is called in the first loop so return from it can be invalid (it can be reported as nonempty when in fact it is). I am not 100% sure that giving a lock here would serve any purpose as in the end there is absolutely no guarantee what will be available when second loop of _sys_mem_pool_block_alloc is called.

To Reproduce
Steps to reproduce the behavior:
Allocate from various context close to no memory condition.

Expected behavior
`'k_malloc' should return NULL if no memory is available.

Impact
Showstopper

Screenshots or console output
N/A

Environment (please complete the following information):
ncs zephyr 745326266d9c32ba3aa67d5af1f727059d9a88e3 (from d9876be upstream)

Additional context
N/A

[pdunaj]

I reviewed the code and it seems the problem can happen even if memory is available. See #14508

[pdunaj]

FYI @andyross

[pdunaj]

Fix pushed

[andyross]

There was an almost identical bug about a year ago, and I thought this was all fixed. May have been reintroduced? I'll take it.

[andyross]

Sorry, reassign back. Didn't notice you had a patch up.