rtpengine: fix unaligned memory access

[linuxmaniac]

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 5.7 crash due to rtpengine module bencode, on aarch64 (raspberry pi) #3444

Description

Make sure the pointers we return from our continuous memory buffer is always 64-bit aligned as it's used not only for strings, but also for structs/objects, and such unaligned memory access is undefined on some archs and flagged as such by ASAN.

From sipwise/rtpengine@ade8100