Check if the fetch is shutting down in resume_dslookup()

Check if the fetch is shutting down in resume_dslookup()

The fetch can be in the shutting down state when resume_dslookup() is
trying to operate on it.

This is also a security issue, because a malicious actor can set up a
name server which delays certain queries in such a way that the fetch
will time out and shut down, which will cause named to crash.

Add a check to see if the fetch has the shutting down attribute set,
and cancel any further operations on it in such case.

A similar bug had been fixed earlier for the resume_qmin() function,
see [GL #966].