Prevent crash when there is no camera device available

If you try to use QAudioInputSelectorControl with the iOS Simulator then
it would crash as there is no camera device available. This prevents a
crash from happening in that case.

Change-Id: I3904f3d299a11a6df21e66a1138a4319cc0b8a5c
Reviewed-by: Andy Nichols <andy.nichols@digia.com>