HardwareSerial: fix begin() lock issue on error path

[pillo79]

If the user supplied a wrong UART number, the begin() method would return without releasing the lock. Add missing UNLOCK() call.

No other unbalanced locks exist in this file.

[CLAassistant]

All committers have signed the CLA.

[me-no-dev]

This default case is here just to make the compiler happy. In fact, you will never get here with wrong port number, because this check has already been made on line 330

[SuGlider]

I just tested the compilation and it doesn't complain about the lack of default: case.
I'll just remove this case from the code to make it clearer.

[SuGlider]

@pillo79 - Thanks for noticing it and for the PR.
As wrote in the review, such default case will never happen because the _uart_nr is tested at the beginning of the function code.
I have removed such case from the switch in order to make the code clearer.

@me-no-dev - It can be merged to the 2.x and 3.0.0 branches.

[pillo79]

@pillo79 - Thanks for noticing it and for the PR. As wrote in the review, such default case will never happen because the _uart_nr is tested at the beginning of the function code.

Sorry for missing that! 😒 Removing the default case is even better, thanks 👍