Fix NRE in EncodingProvider

src/libraries/System.Private.CoreLib/src/System/Text/EncodingProvider.cs

@@ -71,7 +71,7 @@ internal static void AddProvider(EncodingProvider provider)
 
                 var newProviders = new EncodingProvider[providers.Length + 1];
                 Array.Copy(providers, newProviders, providers.Length);
-                providers[^1] = provider;
+                newProviders[^1] = provider;
 
                 if (Interlocked.CompareExchange(ref s_providers, newProviders, providers) == providers)
                 {

src/libraries/System.Runtime/tests/System/Text/EncodingTests.cs

@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.Linq;
+using Microsoft.DotNet.RemoteExecutor;
 using Moq;
 using Xunit;
 
@@ -106,6 +107,25 @@ public void GetEncodings_FromProvider_DoesNotContainDisallowedEncodings(string e
             });
         }
 
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public void RegisterProvider_EncodingsAreUsable()
+        {
+            RemoteExecutor.Invoke(() =>
+            {
+                for (int i = 0; i < 10; i++)
+                {
+                    Encoding.RegisterProvider(new NullEncodingProvider());
+                    Assert.Same(Encoding.UTF8, Encoding.GetEncoding(65001));
+                }
+            }).Dispose();
+        }
+
+        private sealed class NullEncodingProvider : EncodingProvider
+        {
+            public override Encoding GetEncoding(int codepage) => null;
+            public override Encoding GetEncoding(string name) => null;
+        }
+
         private sealed class ThreadStaticEncodingProvider : EncodingProvider
         {
             private static readonly object _globalRegistrationLockObj = new object();