Fix many more comiler warnings

Most warnings were harmless, some real bugs. Test set should now compile
cleanly with ack, gcc, and clang.

lib/libddekit/src/usb_server.c

@@ -546,7 +546,7 @@ device_connect_callback
 			break;
 	}
 
-	if (_devices[i].dev!= NULL) {
+	if (i >= MAX_DEVS) {
 		DEBUG_MSG("Too much devices...");
 	} else {
 		_devices[i].dev = dev;