fix#10439. see https://github.com/redis/redis/pull/9872
When executing SHUTDOWN we pause the client so we can un-pause it
if the shutdown fails.
this could happen during the timeout, if the shutdown is aborted, but could
also happen from withing the initial `call()` to shutdown, if the rdb save fails.
in that case when we return to `call()`, we'll crash if `c->cmd` has been set to NULL.
The call stack is:
```
unblockClient(c)
replyToClientsBlockedOnShutdown()
cancelShutdown()
finishShutdown()
prepareForShutdown()
shutdownCommand()
```
what's special about SHUTDOWN in that respect is that it can be paused,
and then un-paused before the original `call()` returns.
tests where added for both failed shutdown, and a followup successful one.
fae5b1a19d