MYST3: Fix for issue #1040, "MYST3: Subtitles show box at end of every line".

This issue, which I could only reproduce by compiling ResidualVM without TTF support, seems to be caused by the subtitle string ending with a '\0' character. This would be correct if it was a C string, but not when it's a Common::String(). This fix simply terminates the loop that reads the subtitle string before it can add the '\0' character.