glx: fix error code when there is no context bound

v2: change all related NULL checks to check against dummyContext
v3: really check for dummyContext *only* when ctx was from
    __glXGetCurrentContext
v4: cover more checks, add dummyBuffer, dummyVtable (Emil)

Signed-off-by: Bernard Kilarski <bernard.r.kilarski@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: "11.2" <mesa-stable@lists.freedesktop.org>

src/glx/glx_pbuffer.c

@@ -328,7 +328,7 @@ GetDrawableAttribute(Display * dpy, GLXDrawable drawable,
        *   the calling thread's current context a GLXBadDrawable error is
        *   generated."
        */
-      if (pdraw == NULL || gc == NULL || gc->currentDpy != dpy ||
+      if (pdraw == NULL || gc == &dummyContext || gc->currentDpy != dpy ||
          (gc->currentDrawable != drawable &&
          gc->currentReadable != drawable)) {
          __glXSendError(dpy, GLXBadDrawable, drawable,