fix test failures

Author: ssg

src/Base32.cs

@@ -185,7 +185,7 @@ public ulong DecodeUInt64(string text)
     public bool TryDecodeUInt64(string text, out ulong number)
     {
         Span<byte> output = stackalloc byte[sizeof(ulong)];
-        if (!TryDecode(text, output, out int bytesWritten))
+        if (!TryDecode(text, output, out _))
         {
             number = 0;
             return false;
@@ -196,7 +196,11 @@ public bool TryDecodeUInt64(string text, out ulong number)
             output.Reverse();
         }
 
-        number = BitConverter.ToUInt64(output[..bytesWritten]);
+        // BitConverter.ToUInt64() specifically requires
+        // the span to be 8 bytes long, so we can't just
+        // use bytesWritten in the decoding here, we
+        // need to pass the whole thing.
+        number = BitConverter.ToUInt64(output);
         return true;
     }
 

test/Base32/Rfc4648Test.cs

@@ -114,7 +114,7 @@ public void Encode_BigEndian_ulong_ReturnsExpectedValues(ulong number, string ex
 
     static ulong reverseBytes(ulong number)
     {
-        var span = BitConverter.GetBytes(number).AsSpan();
+        Span<byte> span = BitConverter.GetBytes(number);
         span.Reverse();
         return BitConverter.ToUInt64(span);
     }