MDEV-28077 'Wrong create options' error with 'big_tables' enabled

The cause of the bug is overflow of uint16 KEY_PART_INFO::length and/or
uint16 KEY_PART_INFO::store_length. The solution is to increase the size
of those variables to the 'uint' type (which is 32-bit long)

sql/structs.h

@@ -71,14 +71,14 @@ typedef struct st_key_part_info {	/* Info about a key part */
   uint  offset;                         /* Offset in record (from 0) */
   uint  null_offset;                    /* Offset to null_bit in record */
   /* Length of key part in bytes, excluding NULL flag and length bytes */
-  uint16 length;
+  uint length;
   /* 
     Number of bytes required to store the keypart value. This may be
     different from the "length" field as it also counts
      - possible NULL-flag byte (see HA_KEY_NULL_LENGTH)
      - possible HA_KEY_BLOB_LENGTH bytes needed to store actual value length.
   */
-  uint16 store_length;
+  uint store_length;
   uint16 key_type;
   uint16 fieldnr;                       /* Fieldnr begins counting from 1 */
   uint16 key_part_flag;                 /* 0 or HA_REVERSE_SORT */