org: Fix potential modified src match-data issue
* lisp/org.el (org-fontify-meta-lines-and-blocks-1): When this is run on
a src block, a "leaky" major mode called in
`org-src-font-lock-fontify-block' can modify the match data.
This is problematic, as the match data already set is important for
font-lock. To protect ourselves from this behaviour, we can wrap
`org-src-font-lock-fontify-block' in `save-match-data' to ensure that
the match data for the src block is conserved.