Fix hard-fault when socket created using accept() is deleted

[sentinelt]

Description

When socket created using accept() is deleted the destructor is called
on TCPSocket which in turn calls close() method which executes
"delete this". This causes hard-fault.

Set _factory_allocated to 0 in destructor call which effectively skips
"delete this" in a close method.

Tested on NUCLEO-F767ZI. Applies both to master and to release-candidate branches.

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[0xc0170]

@ARMmbed/mbed-os-ipcore Please review

[SeppoTakalo]

This fix might be correct, but before I can approve, I need to verify it with a testcase.

TCPSocket::accept() is lacking lot of testcases, which are now under development within this sprint. So once we can test this, I will reply back.

[0xc0170]

Set as "preceding PR" needed, should be tested by #8499 ?

[michalpasztamobica]

@0xc0170 , I think that #8499 makes this pull request unnecessary.
@SeppoTakalo , should it be rejected/closed?
@sentinelt , please take a look at #8499 and let us know your thoughts.

[0xc0170]

OK waiting for confirmation

[SeppoTakalo]

Yes, this can be closed.
We will fix the root cause issue, provide a testcase for it and submit all in #8499