cam: options: fix access to uninit variable

operator[] doesn't check if the option exists in the values_ map, so it
can return a pointer to location outside the map.

Fix by returning an empty OptionValue if the option is not found.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@iki.fi>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
[Kieran: Adjust s_empty naming to 'empty']
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/cam/options.cpp

@@ -61,7 +61,12 @@ bool OptionsBase<T>::isSet(const T &opt) const
 template<typename T>
 const OptionValue &OptionsBase<T>::operator[](const T &opt) const
 {
-	return values_.find(opt)->second;
+	static const OptionValue empty;
+
+	auto it = values_.find(opt);
+	if (it != values_.end())
+		return it->second;
+	return empty;
 }
 
 template<typename T>