agent: Fix wrong use of gcry_sexp_build_array

* findkey.c (agent_public_key_from_file): Fix use of
gcry_sexp_build_array.

A test case leading to a segv in Libgcrypt is

gpg-connect-agent \
  "READKEY 9277C5875C8AFFCB727661C18BE4E0A0DEED9260" /bye

The keygrip was created by "monkeysphere s", which has a comment.

gcry_sexp_build_array expects pointers to the arguments which is quite
surprising. Probably ARG_NEXT was accidentally implemented wrongly.
Anyway, we can't do anything about it and thus need to fix the check
the users of this function.

• Some-comments-by: Werner Koch <wk@gnupg.org>