cube bug accepting -.1

The lex scanner for the cube package has an apparent error in its
pattern for matching floating point numbers. It doesn't except cases
where there is no digit between a sign and a decimal point. For example
-.1 is not matched by the pattern.

I have attached a patch to cubeparse.l and the regression differences.

P.S. I still plan to submit enhancement patches for cube before the
end of the week, but as this is a bug fix I thought it should really
go in separately.