Improve string delection in function ContainsString

Pick up patch from Samba:

    commit 5740a06b320d56840439fb7fe3b8b27f341ddaa7
    Author: Matthieu Patou <mat@matws.net>
    Date:   Sat Oct 4 21:19:12 2014 -0700

    pidl: Improve string delection in function ContainsString

Change-Id: I702c12e4275315511728d5ef6db07e03be9c2b38
Reviewed-on: https://code.wireshark.org/review/5091
Reviewed-by: Guy Harris <guy@alum.mit.edu>