Fix ancient memory leak in contrib/auto_explain.

The ExecutorEnd hook is invoked in a context that could be quite
long-lived, not the executor's own per-query context as I think
we were sort of assuming.  Thus, any cruft generated while producing
the EXPLAIN output could accumulate over multiple queries.  This can
result in spectacular leakage if log_nested_statements is on, and
even without that I'm surprised nobody complained before.

To fix, just switch into the executor's context so that anything we
allocate will be released when standard_ExecutorEnd frees the executor
state.  We might as well nuke the code's retail pfree of the explain
output string, too; that's laughably inadequate to the need.

Japin Li, per report from Jeff Janes.  This bug is old, so
back-patch to all supported branches.

Discussion: https://postgr.es/m/CAMkU=1wCVtbeRn0s9gt12KwQ7PLXovbpM8eg25SYocKW3BT4hg@mail.gmail.com

contrib/auto_explain/auto_explain.c

@@ -371,8 +371,15 @@ explain_ExecutorEnd(QueryDesc *queryDesc)
 {
 	if (queryDesc->totaltime && auto_explain_enabled())
 	{
+		MemoryContext oldcxt;
 		double		msec;
 
+		/*
+		 * Make sure we operate in the per-query context, so any cruft will be
+		 * discarded later during ExecutorEnd.
+		 */
+		oldcxt = MemoryContextSwitchTo(queryDesc->estate->es_query_cxt);
+
 		/*
 		 * Make sure stats accumulation is done.  (Note: it's okay if several
 		 * levels of hook all do this.)
@@ -424,9 +431,9 @@ explain_ExecutorEnd(QueryDesc *queryDesc)
 					(errmsg("duration: %.3f ms  plan:\n%s",
 							msec, es->str->data),
 					 errhidestmt(true)));
-
-			pfree(es->str->data);
 		}
+
+		MemoryContextSwitchTo(oldcxt);
 	}
 
 	if (prev_ExecutorEnd)