number of keys given: must be non-negative, small enough that it is
not truncated when stuffed into an int16_t for kadm5_free_key_data,
and small enough to avoid integer overflow when calculating the memory
required for the keys themselves.
XXX Why does kadm5_free_key_data use int16_t?
git-svn-id: svn://svn.h5l.se/heimdal/trunk/heimdal@11415 ec53bebd-3082-4978-b11e-865c3cabbd6b