[#1351] apemanager: Fix AddChain handler for audit middleware

* `GetChainID` from `frostfs-api-go/v2` does not handler nil-pointer
  response body that leads to the panic after its call.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

pkg/services/apemanager/audit.go

@@ -33,10 +33,15 @@ func (a *auditService) AddChain(ctx context.Context, req *apemanager.AddChainReq
 		return res, err
 	}
 
+	var respChainID []byte
+	if respBody := res.GetBody(); respBody != nil {
+		respChainID = respBody.GetChainID()
+	}
+
 	audit.LogRequest(a.log, ape_grpc.APEManagerService_AddChain_FullMethodName, req,
 		audit.TargetFromChainID(req.GetBody().GetTarget().GetTargetType().String(),
 			req.GetBody().GetTarget().GetName(),
-			res.GetBody().GetChainID()),
+			respChainID),
 		err == nil)
 
 	return res, err