Fixed that negative arguments to certain integer options wrap around.


mysql-test/r/variables.result:
  Added a test case for bug#6958.
mysql-test/t/variables.test:
  Added a test case for bug#6958.
sql/set_var.cc:
  sys_var_long_ptr::check function was added.
sql/set_var.h:
  Use sys_var_long_ptr::check function for sys_var_long_ptr class.