allocator: fix panic when accessing nil store descriptor

Previously allocator was trying to update store descriptor counters
on unitialized stores. Instead of checking if detail is nil which
should never happen, it should check if descriptor in detail is not
nil as it is only eventually populated by gossip.

Release note: None

pkg/kv/kvserver/allocator/storepool/store_pool.go

@@ -620,14 +620,14 @@ func (sp *StorePool) UpdateLocalStoreAfterRelocate(
 
 	updateTargets := func(targets []roachpb.ReplicationTarget) {
 		for _, target := range targets {
-			if toDetail := sp.GetStoreDetailLocked(target.StoreID); toDetail != nil {
+			if toDetail := sp.GetStoreDetailLocked(target.StoreID); toDetail.Desc != nil {
 				toDetail.Desc.Capacity.RangeCount++
 			}
 		}
 	}
 	updatePrevious := func(previous []roachpb.ReplicaDescriptor) {
 		for _, old := range previous {
-			if toDetail := sp.GetStoreDetailLocked(old.StoreID); toDetail != nil {
+			if toDetail := sp.GetStoreDetailLocked(old.StoreID); toDetail.Desc != nil {
 				toDetail.Desc.Capacity.RangeCount--
 			}
 		}