Avoid useless iteration on all pages in wxBookCtrlBase::DoGetBestSize().

Avoid useless iteration on all pages in wxBookCtrlBase::DoGetBestSize().

If m_fitToCurrentPage is true, there is no need to iterate over all the pages
computing their max best size only in order to overwrite it with the best size
of the current page later.

This doesn't result in any changes in the behaviour, just avoids useless best
size computations.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72561 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775