Improve HostConfiguration consistency check (paritytech#3897)

fixes paritytech#3886

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

polkadot/runtime/parachains/src/configuration.rs

@@ -187,7 +187,7 @@ pub struct HostConfiguration<BlockNumber> {
 	///
 	/// Must be at least 1.
 	pub no_show_slots: u32,
-	/// The number of delay tranches in total.
+	/// The number of delay tranches in total. Must be at least 1.
 	pub n_delay_tranches: u32,
 	/// The width of the zeroth delay tranche for approval assignments. This many delay tranches
 	/// beyond 0 are all consolidated to form a wide 0 tranche.
@@ -247,7 +247,7 @@ impl<BlockNumber: Default + From<u32>> Default for HostConfiguration<BlockNumber
 			max_validators: None,
 			dispute_period: 6,
 			dispute_post_conclusion_acceptance_period: 100.into(),
-			n_delay_tranches: Default::default(),
+			n_delay_tranches: 1,
 			zeroth_delay_tranche_width: Default::default(),
 			needed_approvals: Default::default(),
 			relay_vrf_modulo_samples: Default::default(),
@@ -315,6 +315,8 @@ pub enum InconsistentError<BlockNumber> {
 	LookaheadExceedsTTL,
 	/// Passed in queue size for on-demand was too large.
 	OnDemandQueueSizeTooLarge,
+	/// Number of delay tranches cannot be 0.
+	ZeroDelayTranches,
 }
 
 impl<BlockNumber> HostConfiguration<BlockNumber>
@@ -412,6 +414,10 @@ where
 			return Err(OnDemandQueueSizeTooLarge)
 		}
 
+		if self.n_delay_tranches.is_zero() {
+			return Err(ZeroDelayTranches)
+		}
+
 		Ok(())
 	}