Fix replacement of event queue background.

[pan-]

Summary of changes

When an existing background function is replaced from the event queue, first reset the existing one to nullptr then set the new one as they share the same memory location.
Otherwise, equeue_background won't terminate the old callback (or worse call into a nullptr).

This was discovered when EventQueue::background was called with a nullptr.
It crashes the software as it tries to call into a nullptr.

Impact of changes

Migration actions required

None

Documentation

None

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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)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

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

[0xc0170]

Should there be test case added to catch this?

[0xc0170]

Styling check failed (empty spaces at the end of lines?)

Pull request has been modified.

[mbed-ci]

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