Process canceled/shut down results in validate_dnskey_dsset_done()

Process canceled/shut down results in validate_dnskey_dsset_done()

When a validator is already shut down, val->name becomes NULL. We
need to process and keep the ISC_R_CANCELED or ISC_R_SHUTTINGDOWN
result code before calling validate_async_done(), otherwise, when it
is called with the hardcoded DNS_R_NOVALIDSIG result code, it can
cause an assetion failure when val->name (being NULL) is used in
proveunsecure().

(cherry picked from commit d85918aebf8e0ce0d19cc44df3bdf04cfae3b475)