Minecarts touching lava crashes the game.

Put the summary of the bug you're having here

What I expected to happen was...
The minecart being destroyed by lava.
What actually happened was...:
The game froze, then crashed.

Steps to Reproduce:
1. Create a minecart system.
2. Place lava on it, obstructing the minecart's path.
3. Get the minecart going, into the lava, crashing the game.