Throw error if resources.PostProcess is used in a deferred template

That just doesn't work.

See #12655

tpl/tplimpl/template_ast_transformers.go

@@ -16,6 +16,7 @@ package tplimpl
 import (
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/gohugoio/hugo/helpers"
 	htmltemplate "github.com/gohugoio/hugo/tpl/internal/go_templates/htmltemplate"
@@ -248,7 +249,12 @@ func (c *templateContext) handleDefer(withNode *parse.WithNode) {
 	n := l.Nodes[0].(*parse.ActionNode)
 
 	inner := withNode.List.CopyList()
-	innerHash := helpers.MD5String(inner.String())
+	s := inner.String()
+	if strings.Contains(s, "resources.PostProcess") {
+		c.err = errors.New("resources.PostProcess cannot be used in a deferred template")
+		return
+	}
+	innerHash := helpers.MD5String(s)
 	deferredID := tpl.HugoDeferredTemplatePrefix + innerHash
 
 	c.deferNodes[deferredID] = inner