Update serial_api.c - Issue #14353 - Setting BufferedSerial pin to NC throws runtime assertion

[mark-psl]

Summary of changes

Fixes Issue #14353 - Fixed issue where setting either Tx/Rx pins to NC for FRDM-K64F target would throw a runtime assertion.

Impact of changes

Code with either Tx or Rx pins set to NC will now work for the FRDM-K64F board.

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]

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

[mbed-ci]

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jenkins-ci/mbed-os-ci_build-greentea-ARM	✔️
jenkins-ci/mbed-os-ci_cmake-example-ARM	✔️
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jenkins-ci/mbed-os-ci_unittests	✔️
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jenkins-ci/mbed-os-ci_build-example-GCC_ARM	✔️
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