xlib: clear currentDpy when releasing the current context

After abe6d750e5, glXDestroyContext() can defer destruction by marking
the context with xid == None while it is still current.

However, the release-current path did not clear current->currentDpy,
so a context that had already been marked for deletion could remain
associated with a display after unbinding.

Fixes: abe6d750e5 ("xlib: fix glXDestroyContext in Gallium frontends")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/14947
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 447a1d2e8d)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40979>

.pick_status.json

@@ -3454,7 +3454,7 @@
         "description": "xlib: clear currentDpy when releasing the current context",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "abe6d750e58d371624de75f4bad365c61e0196c1",
         "notes": null

src/gallium/frontends/glx/xlib/glx_api.c

@@ -1236,6 +1236,8 @@ glXMakeContextCurrent( Display *dpy, GLXDrawable draw,
    }
    else if (!ctx && !draw && !read) {
       /* release current context w/out assigning new one. */
+      if (current)
+         current->currentDpy = NULL;
       XMesaMakeCurrent2( NULL, NULL, NULL );
       SetCurrentContext(NULL);
       return True;