glx: zero out drawable structs after allocation

src/glx/dri2_glx.c

    if (!pdraw)
       return NULL;
 
+   memset(pdraw, 0, sizeof *pdraw);
    pdraw->base.destroyDrawable = dri2DestroyDrawable;
    pdraw->base.xDrawable = xDrawable;
    pdraw->base.drawable = drawable;

src/glx/dri_glx.c

    if (!pdp)
       return NULL;
 
+   memset(pdp, 0, sizeof *pdp);
    pdp->base.drawable = drawable;
    pdp->base.psc = &psc->base;
 

src/glx/drisw_glx.c

    if (!pdp)
       return NULL;
 
+   memset(pdp, 0, sizeof *pdp);
    pdp->base.xDrawable = xDrawable;
    pdp->base.drawable = drawable;
    pdp->base.psc = &psc->base;