When running javascript: URIs in chrome, fall back on a null principal instead

of crashing.  Bug 328697, r=mrbkap, sr=jst

dom/src/jsurl/nsJSProtocolHandler.cpp

@@ -251,6 +251,10 @@ nsresult nsJSThunk::EvaluateScript(nsIChannel *aChannel)
         if (principal) {
             nsCOMPtr<nsIURI> uri;
             rv = principal->GetURI(getter_AddRefs(uri));
+            if (!uri) {
+                rv = NS_ERROR_NOT_AVAILABLE;
+            }
+            
             if (NS_SUCCEEDED(rv)) {
                 nsCAutoString spec;
                 uri->GetSpec(spec);