If `dns_dnssec_keyfromrdata` failed we don't need to call
`dst_key_free` because no `dstkey` was created. Doing so
nevertheless will result in an assertion failure.
This can happen if the key uses an unsupported algorithm.
(cherry picked from commit 7a1ca39b950b7d5230b605ac60f15a1cb94e3d69)