libblkid: fix and cleanup blkid_safe_string()

libblkid: fix and cleanup blkid_safe_string()

* Don't use 'size_t len' variable for utf8_encoded_valid_unichar()
  return code as it returns negative numbers on invalid utf8 sequence.

* Don't rely only on \0, but check the current position against output
  buffer size.

* Accept hex encoding like the original version.

* Use else-if to avoid unnecessary 'continue'.

Signed-off-by: Karel Zak <kzak@redhat.com>