fixed bug that was causing memory corruption in PA/ALSA when host sample format was lager than user format. e.g. using 16 bit output with a sound card that used 24 bit buffers. the incorrect input zeroer was being used. Thanks to Anders Tornvig for identifying the problem

git-svn-id: http://subversion.assembla.com/svn/portaudio/portaudio/trunk@1896 0f58301d-fd10-0410-b4af-bbb618454e57