Do not crash in BlockContainer.CheckInvariant

ICSharpCode.Decompiler/IL/Instructions/BlockContainer.cs

@@ -179,8 +179,8 @@ namespace ICSharpCode.Decompiler.IL
 		internal override void CheckInvariant(ILPhase phase)
 		{
 			base.CheckInvariant(phase);
-			Debug.Assert(EntryPoint == Blocks[0]);
-			Debug.Assert(!IsConnected || EntryPoint.IncomingEdgeCount >= 1);
+			Debug.Assert(Blocks.Count > 0 && EntryPoint == Blocks[0]);
+			Debug.Assert(!IsConnected || EntryPoint?.IncomingEdgeCount >= 1);
 			Debug.Assert(Blocks.All(b => b.HasFlag(InstructionFlags.EndPointUnreachable)));
 			Debug.Assert(Blocks.All(b => b.Kind == BlockKind.ControlFlow)); // this also implies that the blocks don't use FinalInstruction
 			Block bodyStartBlock;