meta: silence declaration after code warning on MinGW

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/mesa/drivers/common/meta_tex_subimage.c

@@ -78,9 +78,10 @@ create_texture_for_pbo(struct gl_context *ctx, bool create_pbo,
       *tmp_pbo = 0;
       buffer_obj = packing->BufferObj;
    } else {
-      assert(create_pbo);
       bool is_pixel_pack = pbo_target == GL_PIXEL_PACK_BUFFER;
 
+      assert(create_pbo);
+
       _mesa_GenBuffers(1, tmp_pbo);
 
       /* We are not doing this inside meta_begin/end.  However, we know the