ExpressionParser: Parse fully qualified control names correctly

Without clearing out the "accumulator" for the backtick parsing,
our control name was full of junk (the previous device name) causing
us to not correctly find the control.

Ensure that always we clear the "accumulator" string during backtick
parsing.

Source/Core/InputCommon/Src/ControllerInterface/ExpressionParser.cpp

@@ -95,6 +95,7 @@ public:
 
 	bool FetchBacktickString(std::string &value, char otherDelim = 0)
 	{
+		value = "";
 		while (it != expr.end())
 		{
 			char c = *it;