pgsql: Fix bad formula in previous commit.

Fix bad formula in previous commit.

Commit d5406dea25b600408e7acf17d5a06e82d3ce6d0d used a slightly
novel, and wrong, approach to compute the length of the last
toast chunk. It worked fine unless the last chunk happened to
have the largest possible size.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5184f110aa4130ec87b0b3e0834292cd8cb1fd8a

Modified Files
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src/backend/access/common/detoast.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)