dri3: Fix MakeCurrent without a default framebuffer

In OpenGL 3.0 and later it is legal to make a context current without
a default framebuffer.

This has been broken since DRI3 support was introduced.

Cc: "13.0 12.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
diff --git a/src/glx/dri3_glx.c b/src/glx/dri3_glx.c
index 358bd5f..4472a0b 100644
--- a/src/glx/dri3_glx.c
+++ b/src/glx/dri3_glx.c
@@ -209,18 +209,24 @@
    struct dri3_context *pcp = (struct dri3_context *) context;
    struct dri3_screen *psc = (struct dri3_screen *) pcp->base.psc;
    struct dri3_drawable *pdraw, *pread;
+   __DRIdrawable *dri_draw = NULL, *dri_read = NULL;
 
    pdraw = (struct dri3_drawable *) driFetchDrawable(context, draw);
    pread = (struct dri3_drawable *) driFetchDrawable(context, read);
 
    driReleaseDrawables(&pcp->base);
 
-   if (pdraw == NULL || pread == NULL)
+   if (pdraw)
+      dri_draw = pdraw->loader_drawable.dri_drawable;
+   else if (draw != None)
       return GLXBadDrawable;
 
-   if (!(*psc->core->bindContext) (pcp->driContext,
-                                   pdraw->loader_drawable.dri_drawable,
-                                   pread->loader_drawable.dri_drawable))
+   if (pread)
+      dri_read = pread->loader_drawable.dri_drawable;
+   else if (read != None)
+      return GLXBadDrawable;
+
+   if (!(*psc->core->bindContext) (pcp->driContext, dri_draw, dri_read))
       return GLXBadContext;
 
    return Success;