Remove nonnull attribute from isc_mem_{get,allocate,reallocate}

Remove nonnull attribute from isc_mem_{get,allocate,reallocate}

The isc_mem_get(), isc_mem_allocate() and isc_mem_reallocate() can
return NULL ptr in case where the allocation size is NULL.  Remove the
nonnull attribute from the functions' declarations.

This stems from the following definition in the C11 standard:

> If the size of the space requested is zero, the behavior is
> implementation-defined: either a null pointer is returned, or the
> behavior is as if the size were some nonzero value, except that the
> returned pointer shall not be used to access an object.

In this case, we return NULL as it's easier to detect errors when
accessing pointer from zero-sized allocation which should obviously
never happen.