buffer: improve copy() performance

There is no need to create a slice when sourceEnd is out of bounds.

PR-URL: https://github.com/nodejs/node/pull/33214


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>