fdo#79384: RTF import: fix literal Shift-JIS text

This is a variable-length encoding, and the second byte may be a RTF
syntax character like \, {, }.

(cherry picked from commit 061190a62fcdbfb3a0b266d5afffbd257a3e692e)

Conflicts:
	writerfilter/source/rtftok/rtfdocumentimpl.cxx
	writerfilter/source/rtftok/rtfdocumentimpl.hxx

Change-Id: I813ccafda18388af3bf05eb7ce9a0253c627b1c4
Reviewed-on: https://gerrit.libreoffice.org/9632
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>