Fixed patch crash on removed objects

The `patch` method crashes when trying to patch nested object that has been removed from the target.

```
> var diff = jsondiffpatch.diff({abc:{value:1}},{abc:{value:2}})
undefined
> jsondiffpatch.patch({}, diff)
TypeError: Cannot read property 'value' of undefined
    at nestedPatchFilter (/Users/g550003/workspace/stackedit/public/res/bower-libs/jsondiffpatch/build/bundle.js:793:46)
    at Pipe.process (/Users/g550003/workspace/stackedit/public/res/bower-libs/jsondiffpatch/build/bundle.js:1080:9)
    at Processor.process (/Users/g550003/workspace/stackedit/public/res/bower-libs/jsondiffpatch/build/bundle.js:1219:12)
    at DiffPatcher.patch (/Users/g550003/workspace/stackedit/public/res/bower-libs/jsondiffpatch/build/bundle.js:208:27)
    at repl:1:16
    at REPLServer.self.eval (repl.js:110:21)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.EventEmitter.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
    at Interface._line (readline.js:531:8)
```