gpgscm: Initialize unused slots in vectors.

* tests/gpgscm/scheme.c (get_vector_object): Initialize unused slots
at the end of vectors.

They should not be used for anything, but let's just initialize them
to something benign to be sure.

• GnuPG-bug-id: T3014
• Signed-off-by: Justus Winter <justus@g10code.com>