panfrost: Fix crash with GALLIUM_HUD

Due to the order of binding shaders, GALLIUM_HUD triggered a NULL pointer
dereference in the new shader variants code.

Fixes: 0fcddd4d2c ("pan/bi: Rework varying linking on Valhall")
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16707>

src/gallium/drivers/panfrost/pan_context.c

@@ -526,6 +526,10 @@ panfrost_update_shader_variant(struct panfrost_context *ctx,
         if (type == PIPE_SHADER_FRAGMENT && !ctx->shader[PIPE_SHADER_VERTEX])
                 return;
 
+        /* Also defer, happens with GALLIUM_HUD */
+        if (!ctx->shader[type])
+                return;
+
         /* Match the appropriate variant */
         signed variant = -1;
         struct panfrost_shader_variants *variants = ctx->shader[type];