Fix a crash when creating XMLWriter with bad file

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3570 dfc29bdd-3216-0410-991c-e03cc46cb475

source/Irrlicht/CFileSystem.cpp

@@ -866,8 +866,12 @@ IXMLReaderUTF8* CFileSystem::createXMLReaderUTF8(IReadFile* file)
 IXMLWriter* CFileSystem::createXMLWriter(const io::path& filename)
 {
 	IWriteFile* file = createAndWriteFile(filename);
-	IXMLWriter* writer = createXMLWriter(file);
-	file->drop();
+	IXMLWriter* writer = 0;
+	if (file)
+	{
+		writer = createXMLWriter(file);
+		file->drop();
+	}
 	return writer;
 }