fix some compiler warnings

Change-Id: I23c490e6228b0fcf656c76ac314c69508ba2329d
diff --git a/twrp-functions.cpp b/twrp-functions.cpp
index 89714dd..b962457 100644
--- a/twrp-functions.cpp
+++ b/twrp-functions.cpp
@@ -66,7 +66,7 @@
 		memset(&buffer, 0, sizeof(buffer));
 		if (fgets(buffer, 128, exec) != NULL) {
 			buffer[128] = '\n';
-			buffer[129] = NULL;
+			buffer[129] = 0;
 			result += buffer;
 		}
 	}