Problem: some valid euc-kr characters were rejected because
condition checking multi-byte tail didn't allow 
multi-byte characters having the second byte in the ranges
[0x41..0x5A]  and [0x61..0x7A].

Fix: allow these byte ranges for mb tails


mysql-test/r/ctype_euckr.result:
  Adding tests
mysql-test/t/ctype_euckr.test:
  Adding tests
strings/ctype-euc_kr.c:
  Fixing wrong tail character pattern