Replaced working with bson utf8 string inside db_mongodb_convert_bson function

[tiglat]

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 Kamailio doesn't start with db_mongodb and usrloc. #1507

Description

During query operations string fields inside result structure db1_res_t could be corrupted because of incorrect usage of bson memory.

This issue is reproduced when we need to read more than 150 documents from the database.

Firstly bson_iter_utf8 function returns pointer to memory which is allocated by bson library. It looks like this memory is used multiple times (some kind of circular buffer). So the data must be copied into Kamailio memory instead storing that pointer. The original pointer becomes invalid after some time.

Secondly, this pointer must not be freed by the caller of bson_iter_utf8. At the current version bson allocated pointer is freed by Kamailio function pkg_free like it is was allocated by pkg_malloc.

[miconda]

Thanks!

I applied a slightly different patch to check the return code of converting the UTF8 value to internal db val with cloning.

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

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