libsysdecode: report invalid cap_rights_t

Previously we'd have an assertion failure in cap_rights_is_set if
sysdecode_cap_rights is called with an invalid cap_rights_t, so test for
validity first.

PR: 222258
Reviewed by: cem
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12391