Fix ArgumentNullException in Decoder.Convert and fix secure hash of ROS<byte>

src/Nerdbank.MessagePack/Extension.cs

@@ -39,17 +39,5 @@ public Extension(sbyte typeCode, ReadOnlyMemory<byte> data)
 
 	/// <inheritdoc/>
 	long IStructuralSecureEqualityComparer<Extension>.GetSecureHashCode()
-	{
-		// We don't have an incremental SipHash implementation, so we have to copy the data to a rented buffer.
-		byte[] rented = ArrayPool<byte>.Shared.Rent(checked((int)this.Data.Length));
-		try
-		{
-			this.Data.CopyTo(rented);
-			return SipHash.Default.Compute(rented.AsSpan(0, (int)this.Data.Length)) + this.TypeCode;
-		}
-		finally
-		{
-			ArrayPool<byte>.Shared.Return(rented);
-		}
-	}
+		=> SipHash.Default.Compute(this.Data) + this.TypeCode;
 }

src/Nerdbank.MessagePack/PolyfillExtensions.cs

@@ -90,6 +90,11 @@ internal static unsafe int GetChars(this Encoding encoding, ReadOnlySpan<byte> s
 
 		internal static unsafe int GetChars(this Encoding encoding, ReadOnlySequence<byte> source, Span<char> destination)
 		{
+			if (source.IsEmpty)
+			{
+				return 0;
+			}
+
 			if (source.IsSingleSegment)
 			{
 				return GetChars(encoding, source.First.Span, destination);
@@ -100,6 +105,11 @@ internal static unsafe int GetChars(this Encoding encoding, ReadOnlySequence<byt
 			bool completed = true;
 			foreach (ReadOnlyMemory<byte> sourceSegment in source)
 			{
+				if (sourceSegment.IsEmpty)
+				{
+					continue;
+				}
+
 				fixed (byte* pSource = sourceSegment.Span)
 				{
 					fixed (char* pDestination = destination)

src/Nerdbank.MessagePack/SecureHash/HashCollisionResistantPrimitives.cs

@@ -287,42 +287,7 @@ private ReadOnlySequenceOfBytesEqualityComparer()
 
 		public override bool Equals(ReadOnlySequence<byte> x, ReadOnlySequence<byte> y) => x.SequenceEqual(y);
 
-		public override long GetSecureHashCode([DisallowNull] ReadOnlySequence<byte> obj)
-		{
-			int segmentCount = 0;
-			foreach (ReadOnlyMemory<byte> segment in obj)
-			{
-				if (++segmentCount > 64)
-				{
-					break;
-				}
-			}
-
-			if (segmentCount <= 64)
-			{
-				Span<long> hashesSpan = stackalloc long[segmentCount];
-				int i = 0;
-				foreach (ReadOnlyMemory<byte> segment in obj)
-				{
-					hashesSpan[i++] = SecureHash(segment.Span);
-				}
-
-				return SipHash.Default.Compute(MemoryMarshal.Cast<long, byte>(hashesSpan));
-			}
-
-			List<long> hashes = [];
-			foreach (ReadOnlyMemory<byte> segment in obj)
-			{
-				hashes.Add(SecureHash(segment.Span));
-			}
-
-#if NET
-			Span<long> span = CollectionsMarshal.AsSpan(hashes);
-#else
-			Span<long> span = hashes.ToArray();
-#endif
-			return SipHash.Default.Compute(MemoryMarshal.Cast<long, byte>(span));
-		}
+		public override long GetSecureHashCode([DisallowNull] ReadOnlySequence<byte> obj) => SipHash.Default.Compute(obj);
 	}
 
 	internal class CollisionResistantEnumHasher<TEnum, TUnderlying>(SecureEqualityComparer<TUnderlying> equalityComparer) : SecureEqualityComparer<TEnum>