Update source to match pull request, see #140

Author: sebastian-lenz

src/td/converter/plugins/ImplementsPlugin.ts

@@ -80,7 +80,9 @@ module td.converter
                 if (target instanceof models.SignatureReflection && target.parameters &&
                     source instanceof models.SignatureReflection && source.parameters) {
                     for (var index = 0, count = target.parameters.length; index < count; index++) {
-                        target.parameters[index].comment.copyFrom(source.parameters[index].comment);
+                        if (target.parameters[index].comment) {
+                            target.parameters[index].comment.copyFrom(source.parameters[index].comment);
+                        }
                     }
                 }
             }