Use assign for deserialize instead of temporary std::string (backport #241)

[mergify]

Description

At deserialize of fixed_string (cdr) or string (fastcdr) temporary std::string was used which may allocate memory dynamically, which is not desirable or needed especially if using fixed_strings.

@Mergifyio backport 1.0.x

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• N/A Any new/modified methods have been properly documented using Doxygen.
• Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• Changes are API compatible.
• N/A New feature has been added to the versions.md file (if applicable).
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: CI pass and failing tests are unrelated with the changes.

---

This is an automatic backport of pull request #241 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 5b8025b has failed:

On branch mergify/bp/1.0.x/pr-241
Your branch is up to date with 'origin/1.0.x'.

You are currently cherry-picking commit 5b8025b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   include/fastcdr/Cdr.h
	both modified:   include/fastcdr/FastCdr.h

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[MiguelCompany]

Looking at this again, I don't think it makes sense to backport to the previous major. Closing.