Added close serial port to device_destruct in unix serial.cc

[kent-williams]

Serial device was not being properly closed on unix systems. This is ok on windows.

[daniel-j-h]

Urgh. Good catch! Can you add an assertion like this one:

sweep-sdk/libsweep/src/unix/serial.cc

Lines 340 to 342 in 6ed0e1d

	if (close(fd) == -1) {
	SWEEP_ASSERT(false && "closing file descriptor during error handling failed");
	}

as not being able to close the serial device will corrupt state - which will lead to the problem you saw here. Then this is ready to go in. Thank you Kent.

[daniel-j-h]

Also this is related to #69, no? If so feel free to merge and close the issue.