json: Fix use of get_context.
* src/gpgme-json.c (create_onetime_context): New. (release_onetime_context): New. (op_sign): Use the new fucntions to create a separate context. (op_encrypt): Use a separate context for key listings. (create_keylist_patterns): Remove unneeded cast.
get_context retruns a static per-process context and can thus not be
used as a separate context. Use dedicated fucntions for this.
- Signed-off-by: Werner Koch <wk@gnupg.org>