resolved: cleanup use of ERRNO_IS_DISCONNECT()

resolved: cleanup use of ERRNO_IS_DISCONNECT()

Given that ERRNO_IS_DISCONNECT() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the argument passed to ERRNO_IS_DISCONNECT() is the value
returned by dns_transaction_emit_tcp() and dns_transaction_emit_udp()
which are not expected to return any positive values, but let's be
consistent anyway and move the ERRNO_IS_DISCONNECT() invocation
to the branch where the return value is known to be negative.