add check data valid in getStringFromFile

so far no check would be crashed when getData faild.
maybe return an empty string is better.

cocos/2d/platform/CCFileUtils.cpp

@@ -547,6 +547,8 @@ static Data getData(const std::string& filename, bool forString)
 std::string FileUtils::getStringFromFile(const std::string& filename)
 {
     Data data = getData(filename, true);
+    if (! data.getBytes())
+    	return "";
     std::string ret((const char*)data.getBytes());
     return ret;
 }