src: unify implementations of Utf8Value etc.

Unify the common code of `Utf8Value`, `TwoByteValue`, `BufferValue`
and `StringBytes::InlineDecoder` into one class. Always make the
result zero-terminated for the first three.

This fixes two problems in passing:

* When the conversion of the input value to String fails,
  make the buffer zero-terminated anyway. Previously, this would
  have resulted in possibly reading uninitialized data in multiple
  places in the code. An instance of that problem can be reproduced
  by running e.g.
  `valgrind node -e 'net.isIP({ toString() { throw Error() } })'`.
* Previously, `BufferValue` copied one byte too much from the source,
  possibly resulting in an out-of-bounds memory access.
  This can be reproduced by running e.g.
  `valgrind node -e \
    'fs.openSync(Buffer.from("node".repeat(8192)), "r")'`.

Further minor changes:
* This lifts the `out()` method of `StringBytes::InlineDecoder`
  to the common class so that it can be used when using the
  overloaded `operator*` does not seem appropiate.
* Hopefully clearer variable names.
* Add checks to make sure the length of the data does not exceed
  the allocated storage size, including the possible null terminator.

PR-URL: https://github.com/nodejs/node/pull/6357
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

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