Cast to unsigned type when interpreting HID descriptor length bytes (…

…libusb 0.1)

The libusb 0.1 interface definition declares a (signed) char type for
control messages.  The HID descriptor length contained within a control
message is intended to be interpreted as a pair of unsigned bytes so
we must cast to uint8_t when doing the arithmetic rather than trip over
the sign bit.

Closes networkupstools#1261, closes networkupstools#1312.

drivers/libusb0.c

@@ -377,7 +377,7 @@ static int libusb_open(usb_dev_handle **udevp,
 
 				upsdebug_hex(3, "HID descriptor, method 1", buf, 9);
 
-				rdlen1 = buf[7] | (buf[8] << 8);
+				rdlen1 = (uint8_t)buf[7] | ((uint8_t)buf[8] << 8);
 			}
 
 			if (rdlen1 < -1) {