Assuming that UncheckedMalloc(0) returns a non-nullptr is non-standard
and we use other allocators as well (e.g., OPENSSL_malloc) that do not
guarantee this behavior. It is the caller's responsibility to check that
size != 0 implies UncheckedMalloc(size) != nullptr, and that's exactly
what the checked variants (Malloc etc.) already do.

The current behavior is also inconsistent with UncheckedRealloc(), which
always returns a nullptr when the size is 0, and with the documentation
in src/README.md as well as with multiple comments in the source code.

This changes UncheckedMalloc(), UncheckedCalloc(), and
UncheckedRealloc() to always return a nullptr when the size is 0 instead
of doing fake allocations in UncheckedMalloc() and UncheckedCalloc()
while returning a nullptr from UncheckedRealloc(). This is consistent
with existing documentation and comments.

Refs: #8571
Refs: #8572
PR-URL: #44543
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>