Fix handling of asterisks in wxPrintf() implementation.

Fix handling of asterisks in wxPrintf() implementation.

Count the number of asterisks before modifying the string we use to do this,
otherwise we were off by one for the format specifications containing two of
them.

This really fixes the handling of asterisks (used for width/precision) in
wxPrintf() format string, it wasn't done correctly by r60120 but now
VsnprintfTestCase::Asterisk() test does pass.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65689 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775