Lexer crash fix

[kivikakk]

Even better than #3899, a straight-up fix.

• Ensure we handle input returning 0 as EOF so as to not trigger an extra YY_INPUT call into nothing.
• Stop using all trailing context; it's equivalent to REJECT and results in an unresizeable buffer.
• Remove the superstitious version number restraint.

/cc @philipturnbull who fuzzed this for me to help point the way.

[lildude]

As with #3846, I trust your C/Flex skills over mine 😄

[kivikakk]

@lildude Thank you! 🙇‍♀️