glx: guard swap-interval functions against destroyed drawables

The GLX specification says about glXDestroyPixmap:

    "The storage for the GLX pixmap will be freed when it is not current
     to any client."

So arguably, functions like glXSwapIntervalMESA can be called after
glXDestroyPixmap has been called for the currently bound GLXPixmap.
In that case, the GLXDRIDrawable no longer exists, and so we just skip
those calls.

Cc: 17.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit f446f3fb33528eebe9b120340fca3ac5c5ba518d)
diff --git a/src/glx/dri3_glx.c b/src/glx/dri3_glx.c
index 2d40f0a..42a94f9 100644
--- a/src/glx/dri3_glx.c
+++ b/src/glx/dri3_glx.c
@@ -564,6 +564,8 @@
 static int
 dri3_set_swap_interval(__GLXDRIdrawable *pdraw, int interval)
 {
+   assert(pdraw != NULL);
+
    struct dri3_drawable *priv =  (struct dri3_drawable *) pdraw;
    GLint vblank_mode = DRI_CONF_VBLANK_DEF_INTERVAL_1;
    struct dri3_screen *psc = (struct dri3_screen *) priv->base.psc;
@@ -597,6 +599,8 @@
 static int
 dri3_get_swap_interval(__GLXDRIdrawable *pdraw)
 {
+   assert(pdraw != NULL);
+
    struct dri3_drawable *priv =  (struct dri3_drawable *) pdraw;
 
   return priv->swap_interval;
diff --git a/src/glx/glxcmds.c b/src/glx/glxcmds.c
index 6c7bbfd..53c9f9c 100644
--- a/src/glx/glxcmds.c
+++ b/src/glx/glxcmds.c
@@ -1761,7 +1761,11 @@
           psc->driScreen->setSwapInterval) {
       __GLXDRIdrawable *pdraw =
 	 GetGLXDRIDrawable(gc->currentDpy, gc->currentDrawable);
-      psc->driScreen->setSwapInterval(pdraw, interval);
+      /* Simply ignore the command if the GLX drawable has been destroyed but
+       * the context is still bound.
+       */
+      if (pdraw)
+         psc->driScreen->setSwapInterval(pdraw, interval);
       return 0;
    }
 #endif
@@ -1807,7 +1811,14 @@
       if (psc && psc->driScreen && psc->driScreen->setSwapInterval) {
          __GLXDRIdrawable *pdraw =
 	    GetGLXDRIDrawable(gc->currentDpy, gc->currentDrawable);
-	 return psc->driScreen->setSwapInterval(pdraw, interval);
+
+         /* Simply ignore the command if the GLX drawable has been destroyed but
+          * the context is still bound.
+          */
+         if (!pdraw)
+            return 0;
+
+         return psc->driScreen->setSwapInterval(pdraw, interval);
       }
    }
 #endif
@@ -1829,7 +1840,8 @@
       if (psc && psc->driScreen && psc->driScreen->getSwapInterval) {
          __GLXDRIdrawable *pdraw =
 	    GetGLXDRIDrawable(gc->currentDpy, gc->currentDrawable);
-	 return psc->driScreen->getSwapInterval(pdraw);
+         if (pdraw)
+            return psc->driScreen->getSwapInterval(pdraw);
       }
    }
 #endif