glx/dri3: guard in_current_context against a disappeared drawable

Cc: 17.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 21ec35566be2c1aca07083a67f462618ae15fa86)
diff --git a/src/glx/dri3_glx.c b/src/glx/dri3_glx.c
index f7bcba3..2d40f0a 100644
--- a/src/glx/dri3_glx.c
+++ b/src/glx/dri3_glx.c
@@ -119,6 +119,10 @@
 glx_dri3_in_current_context(struct loader_dri3_drawable *draw)
 {
    struct dri3_drawable *priv = loader_drawable_to_dri3_drawable(draw);
+
+   if (!priv)
+      return false;
+
    struct dri3_context *pcp = (struct dri3_context *) __glXGetCurrentContext();
    struct dri3_screen *psc = (struct dri3_screen *) priv->base.psc;