BLAKE2b.cs: Check whether the stream can be read.

stream.Read() throws NotSupportedException, but this is more consistent with the rest of the library and prevents allocations.

src/Geralt.Tests/BLAKE2bTests.cs

@@ -189,6 +189,10 @@ public void ComputeHash_Stream_Invalid(int hashSize, int messageSize)
         if (messageSize > 0) {
             using var m = new MemoryStream(new byte[messageSize], writable: false);
             Assert.ThrowsException<ArgumentOutOfRangeException>(() => BLAKE2b.ComputeHash(h, m));
+
+            h = new byte[BLAKE2b.HashSize];
+            m.Close();
+            Assert.ThrowsException<InvalidOperationException>(() => BLAKE2b.ComputeHash(h, m));
         }
         else {
             using MemoryStream? m = null;

src/Geralt/Crypto/BLAKE2b.cs

@@ -30,6 +30,7 @@ public static void ComputeHash(Span<byte> hash, Stream message)
     {
         Validation.SizeBetween(nameof(hash), hash.Length, MinHashSize, MaxHashSize);
         Validation.NotNull(nameof(message), message);
+        if (!message.CanRead) { throw new InvalidOperationException("The stream cannot be read."); }
         int bytesRead;
         Span<byte> buffer = new byte[StreamBufferSize];
         using var blake2b = new IncrementalBLAKE2b(hash.Length);