Some exceptions fixed

Fixed two exceptions:

* Commit 2b5487f5a5107095ee617e827886ea75e069772d

```
assert.js:102
  throw new assert.AssertionError({
        ^
AssertionError: false == true
    at Timer.sendPosition (~/src/mineflayer/lib/plugins/physics.js:246:12)
    at Timer.timer.ontimeout (timers.js:242:14)
    at process._makeCallback (node.js:318:24)
```

* Commit cc3eae10f622da24c9051268e9fc8ec3fe01ed7e (fixing https://github.com/superjoe30/mineflayer/issues/146)

```
buffer.js:577
    throw new RangeError('Trying to access beyond buffer length');
    ^
RangeError: Trying to access beyond buffer length
    at checkOffset (buffer.js:577:11)
    at Buffer.readUInt8 (buffer.js:583:5)
    at nib (~/src/mineflayer/lib/plugins/blocks.js:132:31)
    at Bot.blockAt (~/src/mineflayer/lib/plugins/blocks.js:115:22)
    at collisionInRange (~/src/mineflayer/lib/plugins/physics.js:187:23)
    at nextFrame (~/src/mineflayer/lib/plugins/physics.js:144:11)
    at Timer.doPhysics (~/src/mineflayer/lib/plugins/physics.js:50:5)
    at Timer.timer.ontimeout (timers.js:242:14)
    at process._makeCallback (node.js:318:24)
```