Add stack depth checks to key recursive functions in backend/nodes/*.c.

Although copyfuncs.c has a check_stack_depth call in its recursion,
equalfuncs.c, outfuncs.c, and readfuncs.c lacked one.  This seems
unwise.

Likewise fix planstate_tree_walker(), in branches where that exists.

Discussion: https://postgr.es/m/30253.1544286631@sss.pgh.pa.us

src/backend/nodes/outfuncs.c

@@ -24,6 +24,7 @@
 #include <ctype.h>
 
 #include "lib/stringinfo.h"
+#include "miscadmin.h"
 #include "nodes/plannodes.h"
 #include "nodes/relation.h"
 #include "utils/datum.h"
@@ -2941,6 +2942,9 @@ _outConstraint(StringInfo str, const Constraint *node)
 static void
 _outNode(StringInfo str, const void *obj)
 {
+	/* Guard against stack overflow due to overly complex expressions */
+	check_stack_depth();
+
 	if (obj == NULL)
 		appendStringInfoString(str, "<>");
 	else if (IsA(obj, List) ||IsA(obj, IntList) || IsA(obj, OidList))