mlpl ParsableString: Fix segmentation fault caused by user-supplied non-ascii strings. [v2]

In SeparatorChecker, an array (m_separatorArray[]) is used for checking if each character is one of separators or not.
The array index is converted from char. Singedness of char is implementation-defined and non-ascii (e.g. utf-8) characters may be evaluated as negative values.
In fact, we screwed up if a user set up an "action" with non-ascii string (ああああ.sh for example).
Note that we can assume that all separators are ascii characters, however, user-supplied string must not result in accessing the array out of bounds.

Signed-off-by: YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>