vmem: make sure a pool is destroyed

Make sure that a destroyed pool cannot be reused.

Ref: pmem/issues#20
Merge pull request #90 from ldorau/vmem-destroy-pool_id-in-pool_destroy

vmem: make sure a pool is destroyed
vmem: remove unnecessary assert in vmem_free()
Merge pull request #91 from krzycz/vmem-null_free

vmem: remove unnecessary assert in vmem_free()