Return a valid partition index also when there's only the disk device.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@21787 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/tests/apps/partitioner/Partitioner.cpp

@@ -549,6 +549,7 @@ private:
 		// if the disk device has no children, we select it without asking
 		if (fDevice->CountChildren() == 0) {
 			partition = fDevice;
+			_partitionIndex = 0;
 			return true;
 		}