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