Check internal context on init context errors

Check internal context on init context errors

If the mechanism deletes the internal context handle on error, the
mechglue must do the same with the union context, to avoid crashes if
the application calls other functions with this invalid union context.

[ghudson@mit.edu: edit commit message and code comment]

ticket: 8337 (new)
target_version: 1.14-next
target_version: 1.13-next
tags: pullup