Fix ip parsing in ipops

[nbruning]

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue dns_int_match_ip is broken #2645

Description

For #2645, a patch was pushed in 47b45b6, but this code does not correctly check the return value of str2ip6buf and str2ipbuf.

As described in the comments on both methods, they return zero for success.

This MR should be backported to 5.4 as well.

[miconda]

You have to format the commit message as per contributing guidelines:

• https://github.com/kamailio/kamailio/blob/master/.github/CONTRIBUTING.md#commit-message-format

Being small changes, I can do the fix manually this time.

[miconda]

Slightly different patch was committed and backported. Thanks!