Don't crash on assigning nil to a string in lua

include/extractor/extraction_way.hpp

@@ -13,6 +13,21 @@ namespace osrm
 {
 namespace extractor
 {
+namespace detail
+{
+    inline void maybeSetString(std::string &str, const char* value)
+    {
+        if (value == nullptr)
+        {
+            str.clear();
+        }
+        else
+        {
+            str = std::string(value);
+        }
+    }
+}
+
 
 /**
  * This struct is the direct result of the call to ```way_function```
@@ -50,6 +65,20 @@ struct ExtractionWay
     void set_backward_mode(const TravelMode m) { backward_travel_mode = m; }
     TravelMode get_backward_mode() const { return backward_travel_mode; }
 
+    // wrappers to allow assigning nil (nullptr) to string values
+    void SetName(const char* value) { detail::maybeSetString(name, value); }
+    const char* GetName() const { return name.c_str(); }
+    void SetRef(const char* value) { detail::maybeSetString(ref, value); }
+    const char* GetRef() const { return ref.c_str(); }
+    void SetDestinations(const char* value) { detail::maybeSetString(destinations, value); }
+    const char* GetDestinations() const { return destinations.c_str(); }
+    void SetPronunciation(const char* value) { detail::maybeSetString(pronunciation, value); }
+    const char* GetPronunciation() const { return pronunciation.c_str(); }
+    void SetTurnLanesForward(const char* value) { detail::maybeSetString(turn_lanes_forward, value); }
+    const char* GetTurnLanesForward() const { return turn_lanes_forward.c_str(); }
+    void SetTurnLanesBackward(const char* value) { detail::maybeSetString(turn_lanes_backward, value); }
+    const char* GetTurnLanesBackward() const { return turn_lanes_backward.c_str(); }
+
     double forward_speed;
     double backward_speed;
     double duration;