MDEV-6838: Using too big key for internal temp tables

This bug manifests due to wrong computation and evaluation of
keyinfo->key_length. The issues were:
* Using table->file->max_key_length() as an absolute value that must not be
  reached for a key, while it represents the maximum number of bytes
  possible for a table key.
* Incorrectly computing the keyinfo->key_length size during
  KEY_PART_INFO creation. The metadata information regarding the key
  such the field length (for strings) was added twice.

sql/structs.h

@@ -76,6 +76,7 @@ typedef struct st_key_part_info {	/* Info about a key part */
   */
   uint16 store_length;
   uint16 key_type;
+  /* Fieldnr begins counting from 1 */
   uint16 fieldnr;			/* Fieldnum in UNIREG */
   uint16 key_part_flag;			/* 0 or HA_REVERSE_SORT */
   uint8 type;