contrib: Fix CVE-2018-12356 by hardening the regex

[codarrenvelvindron]

Detailed write-up here:
https://neopg.io/blog/pass-signature-spoof/

[UdjinM6]

Thanks! 👍

However, master is considered to be a stable branch and we do not merge into it directly https://github.com/dashpay/dash#development-process. Pls submit this patch to develop branch. You can either rebase this PR on top of develop branch dropping all unrelated commits, force push and change base branch of this PR in github GUI OR you can simply create new PR (start new branch from develop this time and cherry-pick the patch).

PS. Same as bitcoin#13479

EDIT: nvm, I submitted it myself.

[UdjinM6]

Closing, see #2126.