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This makes the logic in UTF16BEToUTF8() match UTF16LEToUTF8().

* encoding.c:
(UTF16LEToUTF8):
- Fix comment to describe what the code does.
(UTF16BEToUTF8):
- Fix undefined behavior which was applied to UTF16LEToUTF8() in
  2f9382033e.
- Add bounds check to while() loop which was applied to
  UTF16LEToUTF8() in be803967db.
- Do not return -2 when (in >= inend) to fix the bug.  This was
  applied to UTF16LEToUTF8() in 496a1cf592.
- Inline (<< 8) statements to match UTF16LEToUTF8().

Add the following tests and results:

  test/text-4-byte-UTF-16-BE-offset.xml
  test/text-4-byte-UTF-16-BE.xml
  test/text-4-byte-UTF-16-LE-offset.xml
  test/text-4-byte-UTF-16-LE.xml