Prevent segfault with empty allow-from-file and allow-from options

[swegener]

Short description

parseACL() returns a nullptr when both options are set empty.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[rgacogne]

Thanks! I wonder if it wouldn't be better to return an empty std::shared<NetmaskGroup> from parseACL instead of nullptr when both directives are empty, so basically to replace return nullptr with return result. Let's see what Otto prefers :)

[swegener]

I've changed the logic to correctly print the warning message, if the options are set empty. It was just avoiding the segfault before.

[omoerbeek]

Thanks! I wonder if it wouldn't be better to return an empty std::shared<NetmaskGroup> from parseACL instead of nullptr when both directives are empty, so basically to replace return nullptr with return result. Let's see what Otto prefers :)

I like a single return result at the end of parseACL better indeed.

[kpfleming]

Oops! Sorry about overlooking this when I redid that code :-)

[swegener]

Updated to return result

[omoerbeek]

I'm a bit sad none of CodeQL, coverity or scanbuild has seen this bug...