agent: Add some extra robustness to extract_private_key

* agent/cvt-openpgp.c (extract_private_key): Add arg "arraysize".
Make sure that R_FLAGS and R_CURVE are set to NULL.

Given that extract_private_key is not file local it is good to have some
extra asserts to protect against future wrong use.

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