Re: Don't pass NULL pointer to strcmp().

I wrote:
> Hmm ... if we're doing it ourselves, I suppose we've got to consider
> it supported :-(. But I'm still wondering how many seldom-used
> code paths didn't get the message. An example here is that this
> could lead to GetConfigOptionResetString returning NULL, which
> I think is outside its admittedly-vague API spec.

After digging around for a bit, I think part of the problem is a lack
of a clearly defined spec for what should happen with NULL string GUCs.
In the attached v3, I attempted to remedy that by adding a comment in
guc_tables.h (which is maybe not the best place but I didn't see a
better one). That led me to a couple more changes beyond what you had.

It's possible that some of these are unreachable --- for example,
given that a NULL could only be the default value, I'm not sure that
the fix in write_one_nondefault_variable is a live bug. But we ought
to code all this stuff defensively, and most of it already was
NULL-safe.

regards, tom lane