Fix possible crash in git branch dialog

Do not crash when checking out a branch while not currently
on any branch.

Task-number: QTCREATORBUG-5784
Change-Id: I5d007e91c1778e6f09a5d543990d948bc1427599
Reviewed-on: http://codereview.qt.nokia.com/2958Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Robert Löhning <robert.loehning@nokia.com>

src/plugins/git/branchmodel.cpp

@@ -448,8 +448,11 @@ void BranchModel::checkoutBranch(const QModelIndex &idx)
     }
     if (m_client->synchronousCheckoutBranch(m_workingDirectory, branch, &errorMessage)) {
         if (errorMessage.isEmpty()) {
-            static_cast<BranchNode *>(currentBranch().internalPointer())->current = false;
-            emit dataChanged(currentBranch(), currentBranch());
+            QModelIndex currentIdx = currentBranch();
+            if (currentIdx.isValid()) {
+                static_cast<BranchNode *>(currentIdx.internalPointer())->current = false;
+                emit dataChanged(currentBranch(), currentBranch());
+            }
             static_cast<BranchNode *>(idx.internalPointer())->current = true;
             emit dataChanged(idx, idx);
         } else {