MYST3: Don't add terminating '\0' to subtitle string. Fixes #1040.

It's not needed when using Common::String, since they keep track
of the length in a separate field, and it caused glitches when
ResidualVM was compiled without TTF support. See issue #1040,
"MYST3: Subtitles show box at end of every line", for an example.
Merge pull request #1043 from eriktorbjorn/myst3-subtitles

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