Buffer used when setting variables was not dimensioned to accomodate
trailing '\0'. An overflow by one character was therefore possible.
CS corrects limits to prevent such overflows.


mysql-test/r/variables.result:
  Try to overflow buffer used for setting system variables.
  Unpatched server should throw a valgrind warning here.
  Actual value and error message irrelevant, only length counts.
mysql-test/t/variables.test:
  Try to overflow buffer used for setting system variables.
sql/set_var.cc:
  Adjust maximum number of characters we can store in 'buff' by one
  as strmake() will write a terminating '\0'.