st/dri: Add support for surfaceless current contexts.

Tested with Wayland.

src/gallium/state_trackers/dri/common/dri_context.c

@@ -178,7 +178,8 @@ dri_make_current(__DRIcontext * cPriv,
       read->texture_stamp = driReadPriv->lastStamp - 1;
    }
 
-   ctx->stapi->make_current(ctx->stapi, ctx->st, &draw->base, &read->base);
+   ctx->stapi->make_current(ctx->stapi, ctx->st,
+         (draw) ? &draw->base : NULL, (read) ? &read->base : NULL);
 
    return GL_TRUE;
 }

src/gallium/state_trackers/dri/common/dri_drawable.h

@@ -70,7 +70,8 @@ struct dri_drawable
 static INLINE struct dri_drawable *
 dri_drawable(__DRIdrawable * driDrawPriv)
 {
-   return (struct dri_drawable *)driDrawPriv->driverPrivate;
+   return (struct dri_drawable *) (driDrawPriv)
+      ? driDrawPriv->driverPrivate : NULL;
 }
 
 /***********************************************************************

src/gallium/state_trackers/dri/common/dri_screen.c

@@ -231,6 +231,9 @@ dri_fill_st_visual(struct st_visual *stvis, struct dri_screen *screen,
 {
    memset(stvis, 0, sizeof(*stvis));
 
+   if (!mode)
+      return;
+
    stvis->samples = mode->samples;
    stvis->render_buffer = ST_ATTACHMENT_INVALID;