sharded: make stop robust against non-started instances
sharded: make stop robust against non-started instances

If we call start, and then fail in the middle of it, it might be
the case that some instances will be started, while some won't.

In that case, we should not call the stop function for the invalid instance.

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@cloudius-systems.com>

flashbuckets/seastar,mixja/seastar,norcimo5/seastar,kjniemi/seastar,mixja/seastar,wildinto/seastar,bowlofstew/seastar,linearregression/seastar,scylladb/seastar,wildinto/seastar,chunshengster/seastar,syuu1228/seastar,flashbuckets/seastar,tempbottle/seastar,syuu1228/seastar,norcimo5/seastar,dwdm/seastar,shyamalschandra/seastar,joerg84/seastar,sjperkins/seastar,xtao/seastar,cloudius-systems/seastar,dwdm/seastar,flashbuckets/seastar,kjniemi/seastar,dreamsxin/seastar,printedheart/seastar,dreamsxin/seastar,sjperkins/seastar,scylladb/scylla-seastar,joerg84/seastar,dreamsxin/seastar,wildinto/seastar,stamhe/seastar,printedheart/seastar,anzihenry/seastar,xtao/seastar,acbellini/seastar,linearregression/seastar,shyamalschandra/seastar,tempbottle/seastar,anzihenry/seastar,stamhe/seastar,acbellini/seastar,xtao/seastar,avikivity/seastar,leejir/seastar,hongliangzhao/seastar,jonathanleang/seastar,syuu1228/seastar,joerg84/seastar,scylladb/scylla-seastar,dwdm/seastar,koolhazz/seastar,jonathanleang/seastar,chunshengster/seastar,koolhazz/seastar,bowlofstew/seastar,scylladb/seastar,avikivity/seastar,sjperkins/seastar,bowlofstew/seastar,hongliangzhao/seastar,acbellini/seastar,tempbottle/seastar,anzihenry/seastar,leejir/seastar,printedheart/seastar,scylladb/scylla-seastar,norcimo5/seastar,shyamalschandra/seastar,cloudius-systems/seastar,scylladb/seastar,linearregression/seastar,stamhe/seastar,avikivity/seastar,chunshengster/seastar,koolhazz/seastar,mixja/seastar,jonathanleang/seastar,kjniemi/seastar,cloudius-systems/seastar,hongliangzhao/seastar,leejir/seastar