pgsql: Use pg_assume() to avoid compiler warning below exec_set_found()

Use pg_assume() to avoid compiler warning below exec_set_found()

The warning, visible when building with -O3 and a recent-ish gcc, is due to
gcc not realizing that found is a byvalue type and therefore will never be
interpreted as a varlena type.

Discussion: /messages/by-id/3prdb6hkep3duglhsujrn52bkvnlkvhc54fzvph2emrsm4vodl@77yy6j4hkemb
Discussion: /messages/by-id/20230316172818.x6375uvheom3ibt2@awork3.anarazel.de
Discussion: /messages/by-id/20240207203138.sknifhlppdtgtxnk@awork3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/48a23f6eae710d2c5c29f38e66d76e7919117e4d

Modified Files
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src/pl/plpgsql/src/pl_exec.c | 9 +++++++++
1 file changed, 9 insertions(+)