decoders writing control-1 chars?

[Ilya N. Golubev]

> debug build of 21.5

The patch was for 21.4, the branch the data mangling was reported for.
Never tried to merge it in 21.5.  Neither have ever investigated the
behavior of ccl `write-multibyte-character' in 21.5, to decide that it
ever needs patch for this feature in the first place.

And in neither branch never called `ccl-execute-on-string'.  Always
investigated behavior of decoding in ccl coding systems directly.  If
in some branch some ccl operations do meaningful things inside ccl
coding system and crash when done by `ccl-execute-on-string',
considering it a bug in ccl implementation in that branch, and never
have hit it before, let alone tried to fix.