Ignore editor DLLs with no file location.

When waiting for newly enabled editor DLLs to load, ignore all DLLs
that do not have a file-system location.

Fixes #353

Change-Id: I546aedbacb02b2a07352781865c5263304664909

Author: Stewart Miles

source/VersionHandlerImpl/src/VersionHandlerImpl.cs

@@ -2349,6 +2349,7 @@ private static bool EnabledEditorDllsLoaded {
         get {
             var loadedAssemblyPaths = new HashSet<string>();
             foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies()) {
+                if (String.IsNullOrEmpty(assembly.Location)) continue;
                 try {
                     var path = Path.GetFullPath(assembly.Location);
                     if (enabledEditorDlls.Contains(path)) {