NIMSignalingAPI.cs中SignalingCreate等方法传参结构体释放不完整问题

[dujianxin]

传参结构体param_c在构建时为channel_ext_分配内存，调用完成后，仅针对param_c释放，未针对channel_ext_释放，存在内存泄露问题，请确认。

        public static NIMSignalingCreateParam_C GetNativeNIMSignalingCreateParam(NIMSignalingCreateParam param)
        {
            NIMSignalingCreateParam_C param_c = new NIMSignalingCreateParam_C();
            if(param!=null)
            {
                param_c.channel_ext_ = Utf8StringMarshaler.GetInstance("").MarshalManagedToNative(param.channel_ext_); 
                param_c.channel_name_= Utf8StringMarshaler.GetInstance("").MarshalManagedToNative(param.channel_name_); 
                param_c.channel_type_ = param.channel_type_;
                
            }
            return param_c;
        }

        public static void SignalingCreate(NIMSignalingCreateParam param, NimSignalingOptCreateHandler cb)
        {
            NIMSignalingCreateParam_C param_c = SignalHelper.GetNativeNIMSignalingCreateParam(param);
            int nSizeOfParam = Marshal.SizeOf(param_c);
            IntPtr param_ptr = Marshal.AllocHGlobal(nSizeOfParam);
            try
            {
                Marshal.StructureToPtr(param_c, param_ptr, false);
                var user_ptr = NimUtility.DelegateConverter.ConvertToIntPtr(cb);
                SignalingNativeMethods.nim_signaling_create(param_ptr, NimSignalingCreateCb, user_ptr);
            }
            catch (Exception)
            {
                cb?.Invoke(NIMSignalingCreateResCode.kAbnormal, null);
            }
            finally
            {
                Marshal.FreeHGlobal(param_ptr);
            }
        }