Fix SentinelGetMasterAddressByName

This is interesting, and may explain some other hangs. Previously, arr was null, and the .Length call in the MultiBulk case was throwing a null ref. This doesn't throw externally through, it just runs forever. While there's a timeout guard against this on the ExecuteSync path, there's not on ExecuteAsync. This is easy to reproduce for example add this at the top of the result processor:

if (result.Type == ResultType.MultiBulk) throw new Exception("Woops");

...and we'll only see timeouts on sync and forever runs on async. I left this code in (commented) for an easy repro case.

/cc @mgravell

StackExchange.Redis.Tests/Sentinel.cs

 using System.IO;
 using System.Net;
 using System.Threading;
+using System.Threading.Tasks;
 using Xunit;
 using Xunit.Abstractions;
 
@@ -62,6 +63,13 @@ public void SentinelGetMasterAddressByNameNegativeTest()
             Assert.Null(endpoint);
         }
 
+        [Fact]
+        public async Task SentinelGetMasterAddressByNameAsyncNegativeTest()
+        {
+            var endpoint = await Server.SentinelGetMasterAddressByNameAsync("FakeServiceName");
+            Assert.Null(endpoint);
+        }
+
         [Fact]
         public void SentinelMasterTest()
         {

StackExchange.Redis/StackExchange/Redis/ResultProcessor.cs

@@ -1414,13 +1414,18 @@ private sealed class SentinelGetMasterAddressByNameProcessor : ResultProcessor<E
         {
             protected override bool SetResultCore(PhysicalConnection connection, Message message, RawResult result)
             {
+                // To repro timeout fail:
+                // if (result.Type == ResultType.MultiBulk) throw new Exception("Woops");
                 switch (result.Type)
                 {
                     case ResultType.MultiBulk:
                         var arr = result.GetItemsAsValues();
 
-                        int port;
-                        if (arr.Length == 2 && int.TryParse(arr[1], out port))
+                        if (result.IsNull)
+                        {
+                            return true;
+                        }
+                        else if (arr.Length == 2 && int.TryParse(arr[1], out var port))
                         {
                             SetResult(message, Format.ParseEndPoint(arr[0], port));
                             return true;