In wc_to_utf8() in windows.c, the zero terminator is written to an invalid
array index, which results in 2 bytes being zeroed in a random place in the
stack. This sometimes causes a crash when running sp_list_ports() (depending
on string length and compiler optimisation settings).
sizeof(wc_str) returns the size in bytes, so cannot be used directly as an
index into that array, it should be divided by sizeof(WCHAR). Otherwise the
zero terminator index is approximately twice what it should be.
This fixes bug #1031.
USB composite devices can contain an ACM serial interface.
On Windows, the correct iSerial descriptor field is assigned to the parent
(composite) device instead of to the actual serial interface. A bogus
value is returned if the serial interface is asked to provide the S/N.
This patch provides a fallback for this kind of device (tested on Android
with adb + cdc gadgets and on Arduino Zero Programming Port)
