rts_evalStableIO: set rtsApiCapability to NULL *before* calling
scheduleWaitThread, matching the way the other eval_* functions do
this.

The previous way lead to a suble race condition:
  - thread A calls rts_evalIO, enters scheduleWaitThread()
    (rtsApiCapability == NULL).
  - thread B calls rts_evalStableIO, creates a main thread and enters
    scheduleWaitThread() (rtsApiCapability == &MainCapability)
  - thread A exits scheduleWaitThread, sees that rtsApiCapability is
    non-NULL, and calls releaseCapability() on it.  This is bogus,
    because thread A doesn't actually hold the capability, and we've
    done a double-release.

This scenario leads to assertion failures in a debug threaded RTS, and
probably crashes in a non-debug threaded RTS.

MERGE TO STABLE