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

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