siptrace: fix drop() usage in siptrace:msg event route

[nferreyra]

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

ctx was not getting initialized with the result of run_top_route() execution preventing drop(); to work in the siptrace:msg event route

[nferreyra]

Hi @miconda. This relates to the siptrace event you've added after #2374. When I was testing I've detected the calling drop(); in the event route was doing nothing. The result of run_top_route() was not being applied to ctx. With this fix it's working as expected
Let me know if there's anything missing.

[miconda]

Thanks!