* charset.c: Integer overflow fixes.

* charset.c: Integer overflow fixes.

Don't rely on undefined behavior with signed left shift overflow.
Don't assume unsigned int fits into fixnum, or that fixnum fits
into unsigned int.  Don't require max_code to be a valid fixnum;
that's not true for gb10830 4-byte on a 32-bit host.  Allow
invalid_code to be a cons, for the same reason.  Require code_offset
to be a character.  Avoid int overflow if max_char is close
to INT_MAX.
(CODE_POINT_TO_INDEX): On 32-bit hosts, return int, not unsigned;
this is intended anyway and avoids some undefined behavior.
(load_charset_map): Pass unsigned, not int, as 2nd arg of
INDEX_TO_CODE_POINT, as that's what it expects.
(Funify_charset, encode_char): Don't stuff unsigned vals into int vars.