Fix #3051: "InvalidOperationException: Stack empty" when decompiling reference assembly with empty method bodies

ICSharpCode.Decompiler/IL/BlockBuilder.cs

@@ -136,7 +136,7 @@ namespace ICSharpCode.Decompiler.IL
 				cancellationToken.ThrowIfCancellationRequested();
 				int start = block.StartILOffset;
 				// Leave nested containers if necessary
-				while (start >= currentContainer.EndILOffset)
+				while (start >= currentContainer.EndILOffset && containerStack.Count > 0)
 				{
 					currentContainer = containerStack.Pop();
 				}

ICSharpCode.Decompiler/IL/ILReader.cs

@@ -462,6 +462,7 @@ namespace ICSharpCode.Decompiler.IL
 				blocksByOffset[0].Block.Instructions.Add(
 					new InvalidBranch("Empty body found. Decompiled assembly might be a reference assembly.")
 				);
+				stackVariables = Enumerable.Empty<ILVariable>();
 				return;
 			}
 			ILParser.SetBranchTargets(ref reader, isBranchTarget);