Fix T89253: template_list allows arbitrary data changes

Blender forbids property changes in .draw() methods. But they weren't
caught after a call to .template_list() with a custom list type.

Support nested calls that disallow writes.

source/blender/python/intern/bpy_rna.c

@@ -8681,6 +8681,8 @@ static int bpy_class_call(bContext *C, PointerRNA *ptr, FunctionRNA *func, Param
       }
 
 #ifdef USE_PEDANTIC_WRITE
+      /* Handle nested draw calls, see: T89253. */
+      const bool rna_disallow_writes_prev = rna_disallow_writes;
       rna_disallow_writes = is_readonly ? true : false;
 #endif
       /* *** Main Caller *** */
@@ -8690,7 +8692,7 @@ static int bpy_class_call(bContext *C, PointerRNA *ptr, FunctionRNA *func, Param
       /* *** Done Calling *** */
 
 #ifdef USE_PEDANTIC_WRITE
-      rna_disallow_writes = false;
+      rna_disallow_writes = rna_disallow_writes_prev;
 #endif
 
       RNA_parameter_list_end(&iter);