(encode_coding_utf_8): Initialize produced_chars to 0.

(encode_coding_utf_8): Initialize produced_chars to 0.
(decode_coding_utf_16): Fix converting high and low bytes to
code-point.
(encode_coding_utf_16): Substitute coding->default_char for
non-Unicode characters.
(decode_coding): Don't call record_insert here.
(setup_coding_system): Initialize `surrogate' of
coding->spec.utf_16 to 0.
(EMIT_ONE_BYTE): Fix for multibyte case.