Fix mlang release after mlang error

* src/mlang-charset.cpp (ansi_charset_to_utf8): Fix release.

In case of errors we may not release the mlang charset anymore
because we are now using it cached. This caused crashes and
probably hangs.

It is a regression from:
59e8a7ee3bcd16275091c9535626e49fc2a6c4af

• GnuPG-Bug-Id: T4141