PyOS_getsig(), PyOS_setsig(): The minimal amount of work to avoid the

PyOS_getsig(), PyOS_setsig(): The minimal amount of work to avoid the
uninitialized memory reads reported in bug #478001.

Note that this doesn't address the following larger issues:

- Error conditions are not documented for PyOS_*sig() in the C API.

- Nothing that actually calls PyOS_*sig() in the core interpreter and
  extension modules actually /checks/ the return value of the call.

Fixing those is left as an exercise for a later day.