json: Don't use strdup but the xtrystrdup wrapper.

* src/gpgme-json.c (create_keylist_patterns): Use CNT as first arg for
xcalloc.
(process_request): s/strdup/xtrystrdup/.

calloc takes two arguments so to be able to detect integer overflow.
Thus if we switch from malloc to calloc we should utilize that.

• Signed-off-by: Werner Koch <wk@gnupg.org>