add null message type, fix crash when null message cames

Author: melode11

src/internal/sio_packet.cpp

@@ -24,6 +24,11 @@ namespace sio
 		val.SetBool(msg.get_bool());
 	}
 
+	void accept_null_message(Value& val)
+	{
+		val.SetNull();
+	}
+
     void accept_int_message(int_message const& msg, Value& val)
     {
         val.SetInt64(msg.get_int());
@@ -105,6 +110,11 @@ namespace sio
 			accept_bool_message(*(static_cast<const bool_message*>(msg_ptr)), val);
 			break;
 		}
+		case message::flag_null:
+		{
+			accept_null_message(val);
+			break;
+		}
         case message::flag_binary:
         {
             accept_binary_message(*(static_cast<const binary_message*>(msg_ptr)), val,doc,buffers);
@@ -177,6 +187,10 @@ namespace sio
 		{
 			return bool_message::create(value.GetBool());
 		}
+		else if(value.IsNull())
+		{
+			return null_message::create();
+		}
         return message::ptr();
     }
 

src/sio_message.h

@@ -27,7 +27,8 @@ namespace sio
             flag_binary,
             flag_array,
             flag_object,
-			flag_boolean
+			flag_boolean,
+			flag_null
         };
 
 		virtual ~message(){};
@@ -113,6 +114,21 @@ namespace sio
         message(flag f):_flag(f){}
     };
 
+	class null_message : public message
+	{
+	protected:
+        null_message()
+			:message(flag_null)
+        {
+        }
+
+	public:
+        static message::ptr create()
+        {
+            return ptr(new null_message());
+        }
+	};
+
 	class bool_message : public message
 	{
 		bool _v;