siputils: walk back from the uri in contact_param_encode() - #4966

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)
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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

Spotted this next to the tm one. The loop should walk back from the uri looking for '<', but with p++ it steps into the uri and stops with q=1. A plain <sip:...> still works since '<' is right before the uri, but I think < sip:...> would get a second pair of brackets. Checked the loop alone on a few header strings, not a full run.