[REVIEW][3-5] i#115684 SfxItemPool performance regression

[Michael Stahl]

the following fixes a performance regression in OOo 3.4;
i have actually fixed this before in OOo times but the fix somehow got
lost in the CWS process overhead... ah, the issue for it was this:

https://issues.apache.org/ooo/show_bug.cgi?id=115684

http://cgit.freedesktop.org/libreoffice/core/commit/?id=7c7d5c0eec4efb95d18b735fb9df4754ba9d8b1f