siptrace: safety check for transaction when checking ACK

[grumvalski]

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 #XXXX (replace XXXX with an open issue number)

Description

Testing with moderate load I had a crash in siptrace, in the newly added check for ACK for negative replies. Analyzing the core, the crash was caused by the invite transaction being 0x0 when t_gett is called after t_lookup_request. I was not able to reproduce it (how can t_lookup_request return 1 and then the transaction being 0x0?), but I've added a check to avoid crashing.

[miconda]

It can be merged.