Fix a bug in the spill stack
Fix a bug in the spill stack

The spill stack was not being initialised correctly when arguments
were passed, nor was the code that managed empty slots in the
spill stack correct.

Signed-off-by: Mark Ryan <88ffc4093c40652fbbf052f026a8b6147509f0aa@gmail.com>

markdryan/subtilis,markdryan/subtilis