Handle TCODK_NONE in wait_for_keypress correctly

Closing the window while waiting for a keypress will cause libtcod to return a
key type of `TCODK_NONE`.

This should be handled correctly instead of panicking. I don't even know why the
assert was there in the first place.

Fixes #28
Merge pull request #38 from tomassedovic/fix-28

Handle TCODK_NONE in wait_for_keypress correctly