MDEV-11465 JSON_LENGTH returns incorrect length.

        Item_func_json_length::val_int fixed.

mysql-test/r/func_json.result

@@ -292,3 +292,12 @@ json_depth('[[], {}]')
 select json_depth('[[[1,2,3],"s"], {}, []]');
 json_depth('[[[1,2,3],"s"], {}, []]')
 4
+select json_length('');
+json_length('')
+NULL
+select json_length('{}');
+json_length('{}')
+0
+select json_length('[1, 2, {"a": 3}]');
+json_length('[1, 2, {"a": 3}]')
+3

mysql-test/t/func_json.test

@@ -129,3 +129,6 @@ select json_depth(cast(NULL as JSON));
 select json_depth('[[], {}]');
 select json_depth('[[[1,2,3],"s"], {}, []]');
 
+select json_length('');
+select json_length('{}');
+select json_length('[1, 2, {"a": 3}]');

sql/item_jsonfunc.cc

@@ -1271,8 +1271,7 @@ longlong Item_func_json_length::val_int()
     return 0;
   }
     
-  return length;
-
+  return length - 1;
 }