Simplest possible fix for null inputStream when ContentLength is 0

Author: wrall

src/System.ServiceModel.Http/src/System/ServiceModel/Channels/HttpResponseMessageHelper.cs

@@ -297,7 +297,10 @@ private async Task<Message> DecodeBufferedMessageAsync(ArraySegment<byte> buffer
             }
             finally
             {
-                inputStream.Dispose();
+                if (inputStream != null)
+                {
+                    inputStream.Dispose();
+                }
             }
         }