- Problem was reported as a SP variable using itself as 
   right value inside SUBSTR caused corruption of data. 
 - This bug could not be verified in either 5.0bk or 5.1bk
 - Added test case to prevent future regressions.


mysql-test/r/sp-vars.result:
  Added test case for a reported regression which couldn't be
  verified.
mysql-test/t/sp-vars.test:
  Added test case for a reported regression which couldn't be
  verified.