Harden script object selection in scriptprefs.js.
Harden script object selection in scriptprefs.js.

Refs #2073

HasClass0/greasemonkey,the8472/greasemonkey,Martii/greasemonkey,Ventero/greasemonkey,HasClass0/greasemonkey,Ventero/greasemonkey,Martii/greasemonkey,the8472/greasemonkey