Fix error handling in gss_export_sec_context()

Fix error handling in gss_export_sec_context()

In the mechglue gss_export_sec_context(), make sure to delete the
union context if the underlying mech context has been deleted.  This
can happen if the mech's gss_export_sec_context() returns a failure
and deletes the context (not a behavior exhibited by any of our
in-tree mechanisms, but an allowed behavior for other mechs), or if we
fail to allocate space for the wrapped token.

[ghudson@mit.edu: commit message; rename exit label to "cleanup" and
make it valid for all exit cases]

ticket: 8240 (new)
target_version: 1.13.3
tags: pullup