st/clear: handle null color renderbuffer

Fixes: 2eb45daa9c ("gallium: de-pointerize pipe_surface")

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35973>

src/mesa/state_tracker/st_cb_clear.c

@@ -406,12 +406,12 @@ st_Clear(struct gl_context *ctx, GLbitfield mask)
          if (b != BUFFER_NONE && mask & (1 << b)) {
             struct gl_renderbuffer *rb
                = ctx->DrawBuffer->Attachment[b].Renderbuffer;
-            enum pipe_format format = _mesa_renderbuffer_get_format(ctx, rb);
             int colormask_index = ctx->Extensions.EXT_draw_buffers2 ? i : 0;
 
             if (!rb || !rb->texture)
                continue;
 
+            enum pipe_format format = _mesa_renderbuffer_get_format(ctx, rb);
             unsigned colormask =
                GET_COLORMASK(ctx->Color.ColorMask, colormask_index);