Fix a crash in STM when unregisterised

Fixes several test failures:
   ../../libraries/stm/tests       2411 [bad exit code] (normal,hpc,profasm,ghci,optllvm)
   ../../libraries/stm/tests       stm046 [bad exit code] (normal,hpc,profasm,ghci,optllvm)
   ../../libraries/stm/tests       stm061 [bad exit code] (normal,hpc,profasm,ghci,optllvm)