regcomp.c: min len is chars, not bytes

regcomp.c: min len is chars, not bytes

The traditionally-called tricky folds occur because, under /i, a
6-byte/3-character sequence can match a 2-byte/1-character sequence.
The code here has assumed that the delta quantity is measured in bytes
(6-2=4), whereas everywhere else (AFAICT), assumes the measure is to be
in characters (3-2=1).