hipMemPtrGetInfo was returning the error hipErrorInvalidValue if it
was called on a nullptr. However, this does not match the malloc
convention where a nullptr has size zero; for example,
malloc_usable_size() returns zero if called on a nullptr.
This commit changes hipMemPtrGetInfo to set the size to zero and
return hipSuccess when called with a nullptr. (This also fits with
hipMalloc and hipFree usage, since hipMalloc of size zero results in a
nullptr, and hipFree of a nullptr is successful.)
JonathanLichtnerAMD
f31afe1d20