Check if code point exceeds maximum. Correctly cast 'char' to 'uint32_t'.

Check if code point exceeds maximum. Correctly cast 'char' to 'uint32_t'.

First fix:
re2c used to check if code point exceeds maximal value for current
encoding when parsing it. When I moved parsing code points to lexer
I forgot the check.

Second fix:
I assumed that 'static_cast<uint32_t>' on 'signed char' zero-extends.
But I was wrong: it sign-extends. Need to cast to 'unsigned char'
instead.