Address undefined behaviour by checking for null before incrementing pointer at end of do/while loop.

[jkeastbb]

This address undefined behaviour found using clang's UBsan:

curl/lib/setopt.c:177:14: runtime error: applying non-zero offset 1 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior curl/lib/setopt.c:177:14 in

[dfandrich]

Does this actually fix a problem? The loop will already exit at the right time without this patch. Is this just a C spec compliance issue?

[jkeastbb]

I haven't observed this actually breaking but the issue was flagged by clang's Undefined Behaviour sanitizer:

`
curl/lib/setopt.c:177:14: runtime error: applying non-zero offset 1 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior curl/lib/setopt.c:177:14 in

`
This change resolves the issue flagged.

[dfandrich]

That looks fine to me then. Would you update the description to mention ubsan?

[jkeastbb]

That looks fine to me then. Would you update the description to mention ubsan?

Okay, thanks, I've done that now. Sorry, I wasn't sure what details were expected. Hope it is okay now.

[dfandrich]

It's the git commit that's the one more people will see when studying the code, rather than the PR description. I've updated that in the same way and committed it. Thanks for the contribution!