zink: assert that image-view format isn't undefined

Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5407>

src/gallium/drivers/zink/zink_surface.c

@@ -94,6 +94,7 @@ zink_create_surface(struct pipe_context *pctx,
    }
 
    ivci.format = zink_get_format(screen, templ->format);
+   assert(ivci.format != VK_FORMAT_UNDEFINED);
 
    // TODO: format swizzles
    ivci.components.r = VK_COMPONENT_SWIZZLE_R;