tools: Fix memory leak in idevicedevmodectl

Thanks to @Fidetro for reporting the issue.
diff --git a/tools/idevicedevmodectl.c b/tools/idevicedevmodectl.c
index 9997cf3..bd1de6a 100644
--- a/tools/idevicedevmodectl.c
+++ b/tools/idevicedevmodectl.c
@@ -389,6 +389,7 @@
 
 	if ((op == OP_ENABLE || op == OP_ARM) && dev_mode_status) {
 		if (dev_mode_status) {
+			idevice_free(device);
 			printf("DeveloperMode is already enabled.\n");
 			return 0;
 		}