_delete_swap()  TypeError if child._parent

In some case (using native-dns) _delete_swap() may throw TypeError 
if child._parent is null. 

Expl:

```
TypeError: Cannot read property '_left' of null
    at Heap._delete_swap (/(...)/native-dns/node_modules/binaryheap/binaryheap.js:202:20)
    at Heap.remove (/(...)/native-dns/node_modules/binaryheap/binaryheap.js:104:10)
    at MemoryStoreExpire.delete (/(...)/native-dns/lib/cache.js:106:19)
    at /(...)/native-dns/lib/memory.js:35:5
    at process.startup.processNextTick.process._tickCallback (node.js:244:9)
````

This minimalist patch prevent this kind of error, but it require a review: is it relevant with the rest of the code ?

Thanks !