Check context handle in gss_export_sec_context()

Check context handle in gss_export_sec_context()

After commit 4f35b27a9ee38ca0b557ce8e6d059924a63d4eff, the
context_handle parameter in gss_export_sec_context() is dereferenced
before arguments are validated by val_exp_sec_ctx_args().  With a null
context_handle, the new code segfaults instead of failing gracefully.
Revert this part of the commit and only dereference context_handle if
it is non-null.

(cherry picked from commit b6f29cbd2ab132e336b5435447348400e9a9e241)

ticket: 8334
version_fixed: 1.13.4
tags: -pullup
status: resolved