pgsql: Fix ExecuteCallStmt to not scribble on the passed-in parsetree.

Fix ExecuteCallStmt to not scribble on the passed-in parse tree.

Modifying the parse tree at execution time is, or at least ought to be,
verboten.  It seems quite difficult to actually cause a crash this way
in v11 (although you can exhibit it pretty easily in HEAD by messing
with plan_cache_mode).  Nonetheless, it's risky, so fix and back-patch.

Discussion: https://postgr.es/m/13789.1541359611@sss.pgh.pa.us

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d358da81414abc57ef44dfd45dfedc3d443e5bd5

Modified Files
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src/backend/commands/functioncmds.c | 23 ++++++++++++++++++-----
1 file changed, 18 insertions(+), 5 deletions(-)