additional cleanups:
- unify decode_blob() and decode_utf() since these two are identical.
  moved the XXX comment to the caller side.
- costified the length argument to decode_blob(); it doesn't have to be
  adjusted.
- removed an unnecessary temporary variable (it didn't seem to improve
  readability)


git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@1664 e5f2f494-b856-4b98-b285-d166d9295462