Git: Add a soft assertion

If directory is empty, buttons remains NULL, which leads to a crash

To reproduce, open Branches dialog then close all editors
and projects and click Log for a branch

Change-Id: Icf06b76c58585c0dd6062017e94e07dad016c275
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>

src/plugins/git/gitclient.cpp

@@ -632,6 +632,7 @@ public:
         m_enableAnnotationContextMenu(enableAnnotationContextMenu),
         m_fileNames(fileNames)
     {
+        QTC_ASSERT(!directory.isEmpty(), return);
         QToolButton *diffButton = addToggleButton(QLatin1String("--patch"), tr("Show Diff"),
                                               tr("Show difference."));
         mapSetting(diffButton, m_client->settings()->boolPointer(GitSettings::logDiffKey));