Alsa: Fix handling of poll descriptors in PaAlsaStream_WaitForFrames().  Otherwise if capture signals before playback a duplex stream could poll the wrong descriptor, causing a loop (reported by Alexander Kartashov).

git-svn-id: http://subversion.assembla.com/svn/portaudio/portaudio/trunk@1897 0f58301d-fd10-0410-b4af-bbb618454e57