MDEV-27203: Valgrind / MSAN errors in Histogram_json_hb::parse_bucket

In read_bucket_endpoint(), handle all possible parser states.

mysql-test/main/statistics_json.result

@@ -4688,7 +4688,7 @@ explain select * from t1_json limit 1;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1_json	ALL	NULL	NULL	NULL	NULL	10	
 Warnings:
-Warning	4186	Failed to parse histogram for table test.t1_json: "size" element not present at offset 28.
+Warning	4186	Failed to parse histogram for table test.t1_json: String or number expected at offset 27.
 UPDATE mysql.column_stats 
 SET histogram='{"histogram_hb":[{"start":"aaa", "size":"not-an-integer"}]}'
 WHERE table_name='t1_json';

sql/opt_histogram_json.cc

@@ -480,6 +480,13 @@ bool read_bucket_endpoint(json_engine_t *je, Field *field, String *out,
   if (json_read_value(je))
     return true;
 
+  if (je->value_type != JSON_VALUE_STRING &&
+      je->value_type != JSON_VALUE_NUMBER)
+  {
+    *err= "String or number expected";
+    return true;
+  }
+
   const char* je_value= (const char*)je->value;
   if (je->value_type == JSON_VALUE_STRING && je->value_escaped)
   {