Ensure process exit-code actuality when it's examined.
Call GET-PROCESS-STATUS-CHANGES if an exit code is requested for
process that isn't known to complete yet (exit-code field is NIL). It
should guarantee that process-exit-code returns a number, not NIL,
after PROCESS-WAIT on a process returned.
Merge branch 'mswinmt' of github.com:akovalenko/sbcl-win32-threads in…