ZipWriter: Write correct EOCD record when more than 65,535 files

0xFFFF will be written to the EOCD to signal to use the ZIP64
CentralDirectory record when the number of files is 65,535 or more.
Fixes #791

src/SharpCompress/Writers/Zip/ZipWriter.cs

@@ -304,7 +304,10 @@ private void WriteEndRecord(ulong size)
 
         // Write normal end of central directory record
         OutputStream.Write(stackalloc byte[] { 80, 75, 5, 6, 0, 0, 0, 0 });
-        BinaryPrimitives.WriteUInt16LittleEndian(intBuf, (ushort)entries.Count);
+        BinaryPrimitives.WriteUInt16LittleEndian(
+            intBuf,
+            (ushort)(entries.Count < 0xFFFF ? entries.Count : 0xFFFF)
+        );
         OutputStream.Write(intBuf.Slice(0, 2));
         OutputStream.Write(intBuf.Slice(0, 2));
         BinaryPrimitives.WriteUInt32LittleEndian(intBuf, sizevalue);