aws-core: fix segfault on late CleanupConfigAndCredentialsCacheManager

[grrtrr]

Empty the std::map on destruction, so that a late destructor call through CleanupConfigAndCredentialsCacheManager does not free elements twice.

Issue #, if available: Resolves #1850.

Description of changes: Empty the std::map in the base class to avoid double-free on second destructor call.

Check all that applies:

• [X] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.) Please refer to #1850 - this was tested to address the described condition.
• [X] Checked if this PR is a breaking (APIs have been changed) change.
• [X] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [] Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

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• [ ] Windows
• [ ] Android
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