Use O_TRUNC for files opened with "w"

Author: waschik

iolib.go

@@ -629,7 +629,7 @@ func ioOpenFile(L *LState) int {
 		mode = os.O_RDONLY
 		writable = false
 	case "w", "wb":
-		mode = os.O_WRONLY | os.O_CREATE
+		mode = os.O_WRONLY | os.O_TRUNC | os.O_CREATE
 		readable = false
 	case "a", "ab":
 		mode = os.O_WRONLY | os.O_APPEND | os.O_CREATE