libfetch: Pass a zeroed digest to DigestCalcResponse.

[jhb]

Summary

GCC 12 warns that passing "" (a constant of char[1]) to a parameter of
type char[33] could potentially overread. It is not clear from the
context that c->qops can never be "auth-int" (and if it can't, then
the "auth-int" handling in DigestCalcResponse is dead code that should
be removed since this is the only place the function is called).

[emaste]

LGTM, hopefully @des can offer some insight on the auth-int issue.

[des]

This is @grembo's code.

[grembo]

In D36825#836513, @des wrote:

This is @grembo's code.

@des Thanks for adding me to the review, but the code for HTTP digest authentication was contributed by Jean-Francois Dockes <[e-mail]> (I contributed code in the area of TLS and certificates).