check Ruby strings for embedded null bytes

The RGSS doesn't do it. But doing it shouldn't be a problem, as it's the
correct way. If a game is broken by this commit (unlikely), then the game
needs to be fixed as silent truncation is highly unlikely to be wanted.
Merge pull request #69 from cremno/check-ruby-string-for-embedded-null-bytes

MRI: check Ruby strings for embedded null bytes