renderer: switch to context 0 before destroy sub

If the current context is one of the subcontext to be destroyed,
vrend_destroy_sub_context() will fail after the last context is
destroyed.

Found thanks to amaerican fuzzy lop.
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

src/vrend_renderer.c

@@ -3870,6 +3870,7 @@ static void vrend_destroy_sub_context(struct vrend_sub_context *sub)
 bool vrend_destroy_context(struct vrend_context *ctx)
 {
    bool switch_0 = (ctx == vrend_state.current_ctx);
+   struct vrend_context *cur = vrend_state.current_ctx;
    struct vrend_sub_context *sub, *tmp;
    if (switch_0) {
       vrend_state.current_ctx = NULL;
@@ -3893,6 +3894,7 @@ bool vrend_destroy_context(struct vrend_context *ctx)
 
    vrend_set_index_buffer(ctx, 0, 0, 0);
 
+   vrend_renderer_force_ctx_0();
    LIST_FOR_EACH_ENTRY_SAFE(sub, tmp, &ctx->sub_ctxs, head)
       vrend_destroy_sub_context(sub);
 
@@ -3902,6 +3904,9 @@ bool vrend_destroy_context(struct vrend_context *ctx)
 
    FREE(ctx);
 
+   if (!switch_0 && cur)
+      vrend_hw_switch_context(cur, true);
+
    return switch_0;
 }