libcacard: don't free sign buffer while sign op is pending

commit 57f97834 cleaned up
the cac_applet_pki_process_apdu function to have a single
exit point. Unfortunately, that commit introduced a bug
where the sign buffer can get free'd and nullified while
it's still being used.

This commit corrects the bug by introducing a boolean to
track whether or not the sign buffer should be freed in
the function exit path.

Signed-off-by: Ray Strode <rstrode@redhat.com>
Reviewed-by: Alon Levy <alon@pobox.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 81b49e8f)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>