pgsql: Fix plan cache issue in PL/pgSQL CALL

Fix plan cache issue in PL/pgSQL CALL

If we are not going to save the plan, then we need to unset expr->plan
after we are done, also in error cases. Otherwise, we get a dangling
pointer next time around.

This is not the ideal solution. It would be better if we could convince
SPI not to associate a cached plan with a resource owner, and then we
could just save the plan in all cases. But that would require bigger
surgery.

Reported-by: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b9986551e0c6129300b9d7a387baf2006724b297

Modified Files
--------------
src/pl/plpgsql/src/expected/plpgsql_call.out | 10 ++++++++++
src/pl/plpgsql/src/pl_exec.c | 30 ++++++++++++++++++++++++----
src/pl/plpgsql/src/sql/plpgsql_call.sql | 4 ++++
3 files changed, 40 insertions(+), 4 deletions(-)