do advance the source pointer over 2 bytes that specify value length.


myisam/mi_key.c:
  Fix for BUG#13814: in mi_pack_key(), when processing NULL value for TINYBLOB key segment,
  do advance the source pointer over 2 bytes that specify value length (for NULL value the 
  format is: null_marker 0x01, value length 0x00 0x00, data: 0x00 keyseg->length times).
  We need to advance over 2 bytes separately as keyseg->length doesn't include them.
mysql-test/r/myisam.result:
  Testcase for BUG#13814
mysql-test/t/myisam.test:
  Testcase for BUG#13814