Profile type not checked in input

[jcoupey]

The current way we parse strings in json input is as follows:

vroom/src/utils/input_parser.cpp

Lines 50 to 53 in 638f6d1

	inline std::string get_string(const rapidjson::Value& object, const char* key) {
	assert(object.HasMember(key));
	return object[key].GetString();
	}

This is currently only used to retrieve the vehicle.profile value (if present). We enforce a check on whether the key exists but not on the actual type before calling GetString. As a result, setting e.g. "profile": 42 for a vehicle hits a rapidjson assertion or crashes.