enc/unicode.c: 'a' is bigger than 'A'

In ASCII, 'a' is bigger than 'A'. Which means 'A' - 'a' is a negative
number (-32, to be precise). In C, the type of 'a' and 'A' are signed
int (cf: ISO/IEC 9899:1990 section 6.1.3.4). So 'A' - 'a' is also a
signed int. It is `(signed int)-32`.

The problem is, OnigCodePoint is unsigned int. Adding a negative
number to a variable of OnigCodepoint (`code` here) introduces an
unintentional cast of `(unsigned)(signed)-32`, which is
4,294,967,264. Adding this value to code then overflows, and the
result eventually becomes normal codepoint.

The series of operations are not a serious problem but because
`code >= 'a'` holds, we can `(code - 'a') + 'A'` to reroute this.

See also: https://github.com/k-takata/Onigmo/pull/107


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65752 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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