storage: avoid tiny race
storage: avoid tiny race

Probably not a problem in practice since `Replica.mu.destroyed` is only
ever written once, but this code still smelled.

paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,veteranlu/cockroach,maxlang/cockroach,glycerine/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,glycerine/cockroach,vaibhavarora/cockroach,glycerine/cockroach,maxlang/cockroach,petermattis/cockroach,petermattis/cockroach,paperstreet/cockroach,veteranlu/cockroach,maxlang/cockroach,petermattis/cockroach,veteranlu/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,vaibhavarora/cockroach,glycerine/cockroach,paperstreet/cockroach,petermattis/cockroach,petermattis/cockroach,veteranlu/cockroach,glycerine/cockroach,paperstreet/cockroach,petermattis/cockroach,glycerine/cockroach,veteranlu/cockroach,paperstreet/cockroach,maxlang/cockroach,petermattis/cockroach,petermattis/cockroach,veteranlu/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,maxlang/cockroach,maxlang/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,paperstreet/cockroach,maxlang/cockroach