Crash on calculateDirection() when allowCornerCutting == false while calculating path through directional tiles

[Teslov-Mikhail]

When using easystar.disableCornerCutting(); app crashes when asked to calculate a path with obstacles that are accessible only from certain directions.

As long as there's at least one easystar.setDirectionalCondition() the app will crash when trying to calculate a path through these tiles.

C:\Users\Teslov\Desktop\openscape\node_modules\easystarjs\src\easystar.js:472
            var direction = calculateDirection(sourceNode.x - x, sourceNode.y - y)
                                                         ^

TypeError: Cannot read property 'x' of undefined
    at isTileWalkable (C:\Users\Teslov\Desktop\openscape\node_modules\easystarjs\src\easystar.js:472:58)
    at calculate (C:\Users\Teslov\Desktop\openscape\node_modules\easystarjs\src\easystar.js:404:26)
    at Socket.<anonymous> (C:\Users\Teslov\Desktop\openscape\index.js:131:14)
    at emitOne (events.js:96:13)
    at Socket.emit (events.js:188:7)
    at C:\Users\Teslov\Desktop\openscape\node_modules\socket.io\lib\socket.js:513:12
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickCallback (internal/process/next_tick.js:98:9)

I would be extremaly glad if someone would find a solution to this problem. This feature is crucial to my project and my expertise with advanced algorithms is lacking.

With corner cutting enabled characters are able to pass through walls.

[prettymuchbryce]

Fixed by #57

Thanks for your help!