Failure when "(" or ")" does not exist in test file.

[touhouEUROBEAT]

When "(" or ")" does not exist in test file, closeParen would continue to increment and causes IndexOutOfBoundException.

[touhouEUROBEAT]

We fixed it by:

1. In findCloseParen, return -1 if closeParen >= markdown.length();
2. getLinks, after line 34, return toReturn if openParen = -1;