Fix thread_sleep_for with zero wake_delay

[evedon]

Summary of changes

If mbed::internal::do_timed_sleep_relative_or_forever(millisec) is called with millisec set to zero, then wake_predicate should be checked for NULL before dereferencing.

Impact of changes

Fixes #12285

Migration actions required

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@evedon, thank you for your changes.
@ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[adbridge]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts