Fix segfault when using --force-tape-width without printer

When force_tape_width is *not* used it is expected to open/close the usb
device.

src/ptouch-print.c

@@ -648,7 +648,7 @@ int main(int argc, char *argv[])
 	if (im != NULL) {
 		gdImageDestroy(im);
 	}
-	if (forced_tape_width > 0) {
+	if (!forced_tape_width) {
 		ptouch_close(ptdev);
 	}
 	libusb_exit(NULL);