Return the usable size of the allocation pointed to by ptr.
The return value may be larger than the size that was requested during allocation.
This function is not a mechanism for in-place
rather it is provided solely as a tool for introspection purposes.
Any discrepancy between the requested allocation size
and the size reported by this function should not be depended on,
since such behavior is entirely implementation-dependent.
ptr must have been allocated by
Jemalloc and must not have been freed yet.