Add reference for CVE-2015-7686 and CVE-2018-12558 upstream mitigation

From the upstream commit:

	> Subject: [PATCH] avoid unnecessary backtrackings
	>
	> Backtracking could cause serious exponential performance issues, luckily
	> that we can avoid it here as BNF of email address is not ambiguous.
	>
	> This fixes CVE-2015-7686 under various $COMMENT_NEST_LEVEL and also
	> CVE-2018-12558

The commit is included in v1.910 upstream.