GLX: don't crash when indirect clients try to set an event mask

After sending the GLXChangeDrawableAttributes request, we also set a
local set of attributes on the DRI drawable.  But in the indirect case
this array won't be present, so skip the setting in that case to avoid a
crash.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

src/glx/glx_pbuffer.c

@@ -137,6 +137,9 @@ ChangeDrawableAttribute(Display * dpy, GLXDrawable drawable,
 #ifdef GLX_DIRECT_RENDERING
    pdraw = GetGLXDRIDrawable(dpy, drawable);
 
+   if (!pdraw)
+      return;
+
    for (i = 0; i < num_attribs; i++) {
       switch(attribs[i * 2]) {
       case GLX_EVENT_MASK: