Fix #15373: match_type XSS vulnerability

Jakub Galczyk discovered[1] a cross site scripting (XSS)
vulnerability in MantisBT 1.2.12 and earlier versions that allows a
malicious person to trick the browser of a target user into executing
arbitrary JavaScript via the URL: search.php?match_type="><script...

This vulnerability is particularly wide reaching due to search.php being
usable by anonymous users on public facing installations of MantisBT (no
user account required).

The value of the "match_type" filter parameter is now correctly
sanitised prior to use in the HTML output displaying the current filter
settings.

[1] http://hauntit.blogspot.de/2013/01/en-mantis-bug-tracker-1212-persistent.html