Don't propertize if the syntax table is unset

`rainbow-delimiters-mode' may be restored via desktop.el. When this
happens inside an `mmm-mode' buffer, the subregion-specific values of
the syntax table will not be restored, and so
`rainbow-delimiters-syntax-table' can be nil. We handle this case by
skipping propertization if the syntax table is unset.

Closes https://github.com/purcell/emacs.d/issues/209