[monodoc] Fix ecma traversal with unattached tree.

Previously the code expected to stop at upper level node using the fact that their element starts with `root:/'. This element is attributed by RootNode when loading a complete tree. However tool like mdoc also loads tree unattached which doesn't create a standard `root:/` top-level root node hence causing the issue.

mcs/class/monodoc/Monodoc/providers/EcmaDoc.cs

@@ -426,8 +426,11 @@ public static char GetNodeMemberTypeChar (Node node)
 		public static int GetNodeLevel (Node node)
 		{
 			int i = 0;
-			for (; !node.Element.StartsWith ("root:/", StringComparison.OrdinalIgnoreCase); i++)
+			for (; !node.Element.StartsWith ("root:/", StringComparison.OrdinalIgnoreCase); i++) {
 				node = node.Parent;
+				if (node == null)
+					return i - 1;
+			}
 			return i - 1;
 		}
 

mcs/class/monodoc/Test/Monodoc/HelpSourceTests.cs

@@ -7,6 +7,7 @@
 
 using Monodoc;
 using Monodoc.Generators;
+using Monodoc.Providers;
 
 // Used by ReachabilityWithCrefsTest
 // using HtmlAgilityPack;
@@ -153,6 +154,19 @@ public void AspNetStyleUrlReachabilityTest ()
 			Assert.IsTrue (rootTree.RenderUrl ("T:System.IComparable{T}", generator, out result), "#6");
 		}
 
+		[Test]
+		public void PublicUrlOnUnattachedHelpSourceRoot ()
+		{
+			// Unattached help source have no root:/ URL attributed
+			var hs = new EcmaHelpSource (Path.Combine (BaseDir, "sources", "netdocs"), false);
+			var rootTree = RootTree.LoadTree (Path.GetFullPath (BaseDir), false);
+			hs.RootTree = rootTree;
+			Assert.IsNull (hs.Tree.RootNode.PublicUrl);
+			var nsChildUrl = hs.Tree.RootNode.ChildNodes.First ().PublicUrl;
+			Assert.IsNotNull (nsChildUrl);
+			StringAssert.StartsWith ("N:", nsChildUrl);
+		}
+
 		/*
 		[Test, Ignore ("Mono documentation is full of syntax errors so we can't use it reliably for this test")]
 		public void ReachabilityWithCrefsTest ()