Fix ZStandardStream.IsZStandard to handle streams with insufficient bytes

Co-authored-by: adamhathcock <527620+adamhathcock@users.noreply.github.com>

src/SharpCompress/Compressors/ZStandard/ZStandardStream.cs

@@ -35,7 +35,12 @@ internal class ZStandardStream : ZstdSharp.DecompressionStream, IStreamStack
     internal static bool IsZStandard(Stream stream)
     {
         var br = new BinaryReader(stream);
-        var magic = br.ReadUInt32();
+        var bytes = br.ReadBytes(4);
+        if (bytes.Length < 4)
+        {
+            return false;
+        }
+        var magic = BitConverter.ToUInt32(bytes, 0);
         if (ZstandardConstants.MAGIC != magic)
         {
             return false;

tests/SharpCompress.Test/Tar/TarArchiveTests.cs

@@ -295,4 +295,42 @@ public class TarArchiveTests : ArchiveTests
 
         Assert.False(isTar);
     }
+
+    [Fact]
+    public void TarArchive_Open_SmallStream_DoesNotCrash()
+    {
+        // Create a small stream with less than 4 bytes (which would crash when checking ZStandard magic bytes)
+        var ms = new MemoryStream(new byte[] { 0x00, 0x01, 0x02 });
+        var options = new ReaderOptions();
+        options.ArchiveEncoding.Default = Encoding.Default;
+
+        // Open the archive (this won't fail immediately as TarArchive.Open doesn't validate)
+        var archive = TarArchive.Open(ms, options);
+
+        // Before the fix, accessing Entries.Count would crash with EndOfFileException when
+        // TarReader.Open tries to check ZStandard format with insufficient bytes
+        // After the fix, this should throw IncompleteArchiveException instead (graceful handling)
+        Assert.Throws<IncompleteArchiveException>(() =>
+        {
+            var count = archive.Entries.Count;
+        });
+    }
+
+    [Fact]
+    public void TarArchive_Open_EmptyStream_DoesNotCrash()
+    {
+        // Create an empty stream
+        var ms = new MemoryStream();
+        var options = new ReaderOptions();
+        options.ArchiveEncoding.Default = Encoding.Default;
+
+        // Open the archive (this won't fail immediately as TarArchive.Open doesn't validate)
+        var archive = TarArchive.Open(ms, options);
+
+        // Before the fix, accessing Entries would crash with EndOfFileException when
+        // TarReader.Open tries to check ZStandard format with insufficient bytes
+        // After the fix, this should handle the empty stream gracefully
+        var entries = archive.Entries;
+        Assert.Empty(entries);
+    }
 }