buffer: throw on failed fill attempts

If fill() attempts to write a string to a buffer, but fails
silently, then uninitialized memory could be leaked. This commit
causes fill() to throw if the string write operation fails.

Refs: https://github.com/nodejs/node/issues/17423
PR-URL: https://github.com/nodejs/node/pull/17427


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>