Security patch for version 5.1.0.4

Taken from a diff of the last commit for
http://plugins.svn.wordpress.org/custom-contact-forms/tags/5.1.0.4/

More info at
http://blog.sucuri.net/2014/08/database-takeover-in-custom-contact-forms
.html
Merge pull request #11 from philipashlock/security-patch

Security patch for version 5.1.0.4