Fix TCPServer::accept()

[juhaylinen]

Description

Missing lock causes "Mutex unlock failed" errors

++ MbedOS Error Info ++
Error Status: 0x80020117 Code: 279 Module: 2
Error Message: Mutex unlock failed
DutThread: Location: 0x0
DutThread: Error Value: 0xFFFFFFFD
Current Thread: Id: 0x20021C20 Entry: 0x80C429D StackSize: 0x1000 StackMem: 0x20021C80 SP: 0x20022AB0
For more info, visit: https://armmbed.github.io/mbedos-error/?error=0x80020117

Pull request type

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

[juhaylinen]

@SeppoTakalo @kjbracey-arm please review

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3060
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8031/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2674
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8031/

[mbed-ci]

Test : SUCCESS

Build number : 2842
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8031/2842