add nullpointer check in LWIP::socket_close

[chrJost]

Summary of changes

Add a nullpointer check for the structure which contains the TCP connection state.

We found that LWIP deletes a structure which contains the current TCP connection state. When calling close() on the corresponding socket the connection state is read, which results in a nullpointer access and subsequent crash.

Impact of changes

Migration actions required

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

[X] 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

As far as we are aware no test covers this case.

Reviewers

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[0xc0170]

CI started

[mbed-ci]

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