Initialize psp->waitX/waitGL for swrast_dri.so.

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=21053 .

src/glx/x11/drisw_glx.c

@@ -405,6 +405,8 @@ driCreateScreen(__GLXscreenConfigs * psc, int screen,
    psp->createContext = driCreateContext;
    psp->createDrawable = driCreateDrawable;
    psp->swapBuffers = driSwapBuffers;
+   psp->waitX = NULL;
+   psp->waitGL = NULL;
 
    return psp;