Fulltext index may get corrupt by certain gbk characters.

The problem was that when skipping leading non-true-word-characters,
we assumed that these characters are always 1 byte long. This is not
the case with gbk character set, since non-true-word-characters may
be 2 bytes long.

Affects 5.0 only.


myisam/ft_parser.c:
  Leading non-true-word-characters may also be multi-byte (e.g. in
  gbk character set).
mysql-test/r/fulltext2.result:
  A test case for BUG#29299.
mysql-test/t/fulltext2.test:
  A test case for BUG#29299.