Fix of NULLIF print statement.

sql/item_cmpfunc.cc

@@ -2826,7 +2826,9 @@ void Item_func_nullif::print(String *str, enum_query_type query_type)
     Therefore, after equal field propagation args[0] and args[2] can point
     to different items.
   */
-  if ((query_type & QT_ITEM_ORIGINAL_FUNC_NULLIF) || args[0] == args[2])
+  if ((query_type & QT_ITEM_ORIGINAL_FUNC_NULLIF) ||
+      (arg_count == 2) ||
+      (args[0] == args[2]))
   {
     /*
       If QT_ITEM_ORIGINAL_FUNC_NULLIF is requested,
@@ -2844,10 +2846,14 @@ void Item_func_nullif::print(String *str, enum_query_type query_type)
       - one "a" for comparison
       - another "a" for the returned value!
     */
-    DBUG_ASSERT(args[0] == args[2] || current_thd->lex->context_analysis_only);
+    DBUG_ASSERT(arg_count == 2 ||
+                args[0] == args[2] || current_thd->lex->context_analysis_only);
     str->append(func_name());
     str->append('(');
-    args[2]->print(str, query_type);
+    if (arg_count == 2)
+      args[0]->print(str, query_type);
+    else
+      args[2]->print(str, query_type);
     str->append(',');
     args[1]->print(str, query_type);
     str->append(')');