Don't use !eStateRunning when you mean eStateStopped in DestroyImpl.

When we go to destroy the process, we first try to halt it, if
we succeeded and the target stopped, we want to clear out the
thread plans and breakpoints in case we still need to resume to complete
killing the process.  If the target was exited or detached, it's
pointless but harmless to do this.  But if the state is eStateInvalid -
for instance if we tried to interrupt the target to Halt it and that
fails - we don't want to keep trying to interact with the inferior,
so we shouldn't do this work.

This change explicitly checks eStateStopped, and only does the pre-resume
cleanup if we did manage to stop the process.