Fix assertion if index is dropped during REFRESH CONCURRENTLY

Heikki Linnakangas <heikki.linnakangas@iki.fi>

Commit: 06f36bc01bf69f888896eb16025db8265b4df9be
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: 2024-02-05T09:03:28Z
Releases: 15.6
Fix assertion if index is dropped during REFRESH CONCURRENTLY

When assertions are disabled, the built SQL statement is invalid and
you get a "syntax error". So this isn't a serious problem, but let's
avoid the assertion failure.

Backpatch to all supported versions.

Reviewed-by: Noah Misch

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