Fix for when memory pre-allocation fails.


mysys/my_alloc.c:
  Added code to null-out the pre-alloc pointer if allocation fails. This is necessary for the next time this procedure is called.
mysql-test/r/variables-big.result:
  Added "big" test for bug fix Bug #27322.
mysql-test/t/variables-big.test:
  Added "big" test for bug fix Bug #27322.