FIX: Buffer overrun in emacs mode header logic

FIX: Buffer overrun in emacs mode header logic

If the emacs mode header is not well-formed (for example, if it is
missing a terminating "-*-"), then we run off the end of our buffer.

The emacs mode header parsing is a little complicated, and the
best answer is probably to clean up the parser. As an easier, quicker,
fix, I simply added a maximum string length check.