Allow size == 0 in isc_mem_{get,allocate,reallocate}

Allow size == 0 in isc_mem_{get,allocate,reallocate}

Calls to jemalloc extended API with size == 0 ends up in undefined
behaviour.  This commit makes the isc_mem_get() and friends calls
more POSIX aligned:

  If size is 0, either a null pointer or a unique pointer that can be
  successfully passed to free() shall be returned.

We picked the easier route (which have been already supported in the old
code) and return NULL on calls to the API where size == 0.