Git: fix crash in branches dialog

When tags are not shown in branches dialog, selecting of any item in
branches list causes crash

Change-Id: I8bfb66202f486c175a14c59f17b43df21ff596ff
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/branchmodel.cpp

@@ -95,6 +95,8 @@ public:
         BranchNode *rn = rootNode();
         if (rn->isLeaf())
             return false;
+        if (root >= rn->children.count())
+            return false;
         return childOf(rn->children.at(root));
     }