Fix deserialization of maps into non-empty maps (backport #340)

[mergify]

Description

As reported in #339, Cdr::deserialize(std::map<K, T>&) presents two problems when the output map is not empty:

• The overload for primitive (arithmetic / enum) mapped types never cleared the output map, so deserializing into an already used map (e.g. a sample reused by DataReader::take_next_sample()) silently kept stale keys and values.
• Both overloads modified the output map (cleared it and/or added elements) before completing the deserialization, so a NotEnoughMemoryException thrown inside the loop left the output map cleared or partially filled.

Following the suggestion in #339, both overloads now deserialize the elements into a new map, which is moved into the output one once the whole deserialization succeeds. This way the previous content of the output map is always replaced, and it is preserved if the deserialization fails.

Two commits:

• First commit adds tests for correct/incorrect deserialization of primitive/non-primitive maps, including the case of reusing the same map for two deserialization calls. The tests fail on current master.
• Second commit contains the fix. All tests pass locally (2118/2118 ctest).

@Mergifyio backport 2.3.x

Fixes #339

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• Regression tests checking the bug and its fix have been added; the added tests pass locally
• 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). Bug fix, no new feature.
• Applicable backports have been included in the description. 2.3.x is affected (reported against 2.3.6).

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.

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This is an automatic backport of pull request #340 done by [Mergify](https://mergify.com).