MDEV-29188 Crash in JSON_EXTRACT

If we have null_value set then decimal/string value/result shoud be 0 pointer.

mysql-test/main/func_json.result

@@ -1005,5 +1005,16 @@ JSON_VALID('{"admin\\"": null}')	{"admin\"": null}
 1	{"\"admin": null}
 1	{"\"": null}
 #
+# MDEV-29188: Crash in JSON_EXTRACT
+#
+CREATE TABLE t1 (j JSON);
+INSERT INTO t1 VALUES
+('{"ID": "4", "Name": "Betty", "Age": 19}'),
+('[10, 20, [30, 40]]');
+SELECT * FROM t1 WHERE JSON_EXTRACT(j, '$.Age')=19;
+j
+{"ID": "4", "Name": "Betty", "Age": 19}
+drop table t1;
+#
 # End of 10.3 tests
 #

mysql-test/main/func_json.test

@@ -613,6 +613,20 @@ SELECT JSON_VALID('{"admin\\"": null}'), '{"admin\\"": null}'
       UNION
       SELECT JSON_VALID('{"\\"": null}'), '{"\\"": null}';
 
+--echo #
+--echo # MDEV-29188: Crash in JSON_EXTRACT
+--echo #
+
+CREATE TABLE t1 (j JSON);
+
+INSERT INTO t1 VALUES
+  ('{"ID": "4", "Name": "Betty", "Age": 19}'),
+  ('[10, 20, [30, 40]]');
+
+SELECT * FROM t1 WHERE JSON_EXTRACT(j, '$.Age')=19;
+
+drop table t1;
+
 --echo #
 --echo # End of 10.3 tests
 --echo #

sql/item_cmpfunc.cc

@@ -795,7 +795,9 @@ int Arg_comparator::compare_e_string()
 {
   String *res1,*res2;
   res1= (*a)->val_str(&value1);
+  DBUG_ASSERT((res1 == NULL) == (*a)->null_value);
   res2= (*b)->val_str(&value2);
+  DBUG_ASSERT((res2 == NULL) == (*b)->null_value);
   if (!res1 || !res2)
     return MY_TEST(res1 == res2);
   return MY_TEST(sortcmp(res1, res2, compare_collation()) == 0);
@@ -832,10 +834,12 @@ int Arg_comparator::compare_decimal()
 {
   my_decimal decimal1;
   my_decimal *val1= (*a)->val_decimal(&decimal1);
+  DBUG_ASSERT((val1 == NULL) == (*a)->null_value);
   if (!(*a)->null_value)
   {
     my_decimal decimal2;
     my_decimal *val2= (*b)->val_decimal(&decimal2);
+    DBUG_ASSERT((val2 == NULL) == (*b)->null_value);
     if (!(*b)->null_value)
     {
       if (set_null)

sql/item_jsonfunc.cc

@@ -1109,12 +1109,14 @@ my_decimal *Item_func_json_extract::val_decimal(my_decimal *to)
       case JSON_VALUE_OBJECT:
       case JSON_VALUE_ARRAY:
       case JSON_VALUE_FALSE:
+      // TODO: fix: NULL should be NULL
       case JSON_VALUE_NULL:
-        break;
+        int2my_decimal(E_DEC_FATAL_ERROR, 0, false/*unsigned_flag*/, to);
+        return to;
     };
   }
-  int2my_decimal(E_DEC_FATAL_ERROR, 0, false/*unsigned_flag*/, to);
-  return to;
+  DBUG_ASSERT(null_value);
+  return 0;
 }