src: fix null deref in AllocatedBuffer::clear

An empty buffer can have a null environment.  Previously, we were
getting away with with this, but -fsanitize=null in clang caught it.

PR-URL: https://github.com/nodejs/node/pull/32892


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>