MDEV-33015: Server crashes upon JSON_SCHEMA_VALID reading NULL from a user

variable

Analysis:
Since the item is NULL, it's json value is NULL but we proceed to parse
it anyway.
Fix:
If json value is NULL, return NULL.

mysql-test/main/func_json.result

@@ -4737,4 +4737,11 @@ NULL
 #
 PREPARE s FROM 'SELECT JSON_SCHEMA_VALID (?,''{}'') FROM DUAL';
 ERROR HY000: Variable schema is not supported.
+#
+# MDEV-33015: Server crashes upon JSON_SCHEMA_VALID reading NULL from a user variable
+#
+SET @a= NULL;
+SELECT JSON_SCHEMA_VALID(@a,'{}');
+JSON_SCHEMA_VALID(@a,'{}')
+NULL
 # End of 11.1 test

mysql-test/main/func_json.test

@@ -3708,4 +3708,12 @@ SELECT JSON_SCHEMA_VALID(NULL, NULL);
 --error ER_JSON_NO_VARIABLE_SCHEMA
 PREPARE s FROM 'SELECT JSON_SCHEMA_VALID (?,''{}'') FROM DUAL';
 
+--echo #
+--echo # MDEV-33015: Server crashes upon JSON_SCHEMA_VALID reading NULL from a user variable
+--echo #
+
+SET @a= NULL;
+SELECT JSON_SCHEMA_VALID(@a,'{}');
+
+
 --echo # End of 11.1 test

sql/item_jsonfunc.cc

@@ -4809,17 +4809,21 @@ bool Item_func_json_schema_valid::fix_length_and_dec(THD *thd)
 
   String *js= NULL;
 
-  if (!is_schema_constant || (null_value= args[0]->null_value))
+  if (!is_schema_constant)
   {
-    if (!is_schema_constant)
-    {
-      my_error(ER_JSON_NO_VARIABLE_SCHEMA, MYF(0));
-    }
+
+    my_error(ER_JSON_NO_VARIABLE_SCHEMA, MYF(0));
     null_value= 1;
     return 0;
   }
+  null_value= args[0]->null_value;
   js= args[0]->val_json(&tmp_js);
 
+  if (!js)
+  {
+    null_value= 1;
+    return 0;
+  }
   json_scan_start(&je, js->charset(), (const uchar *) js->ptr(),
                   (const uchar *) js->ptr() + js->length());
   if (!create_object_and_handle_keyword(thd, &je, &keyword_list,