Correct relation size estimate with low fillfactor

Since commit 29cf61ade3, table_block_relation_estimate_size() considers
fillfactor when estimating number of rows in a relation before the first
ANALYZE. The formula however did not consider tuples may be larger than
available space determined by fillfactor, ending with density 0. This
ultimately means the relation was estimated to contain a single row.

The executor however places at least one tuple per page, even with very
low fillfactor values, so the density should be at least 1. Fixed by
clamping the density estimate using clamp_row_est().

Reported by Heikki Linnakangas. Fix by me, with regression test inspired
by example provided by Heikki.

Backpatch to 17, where the issue was introduced.

Reported-by: Heikki Linnakangas
Backpatch-through: 17
Discussion: https://postgr.es/m/2bf9d973-7789-4937-a7ca-0af9fb49c71e@iki.fi

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