src: fix compiler warnings in node_buffer.cc

The variables could be initialized to `0` to avoid the warnings from
`-Wmaybe-uninitialized`.

Fixes: https://github.com/nodejs/node/issues/38718

PR-URL: https://github.com/nodejs/node/pull/38722


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>