Fix reachable Debug.Assert in SubReadStream.Read when seeked past end

src/libraries/System.IO.Compression/src/System/IO/Compression/ZipCustomStreams.cs

@@ -334,7 +334,7 @@ public override int Read(byte[] buffer, int offset, int count)
             if (_superStream.Position != _positionInSuperStream)
                 _superStream.Seek(_positionInSuperStream, SeekOrigin.Begin);
             if (_positionInSuperStream + count > _endInSuperStream)
-                count = (int)(_endInSuperStream - _positionInSuperStream);
+                count = (int)Math.Max(0L, _endInSuperStream - _positionInSuperStream);
 
             Debug.Assert(count >= 0);
             Debug.Assert(count <= origCount);
@@ -357,7 +357,7 @@ public override int Read(Span<byte> destination)
             if (_superStream.Position != _positionInSuperStream)
                 _superStream.Seek(_positionInSuperStream, SeekOrigin.Begin);
             if (_positionInSuperStream + count > _endInSuperStream)
-                count = (int)(_endInSuperStream - _positionInSuperStream);
+                count = (int)Math.Max(0L, _endInSuperStream - _positionInSuperStream);
 
             Debug.Assert(count >= 0);
             Debug.Assert(count <= origCount);
@@ -395,7 +395,7 @@ async ValueTask<int> Core(Memory<byte> buffer, CancellationToken cancellationTok
 
                 if (_positionInSuperStream > _endInSuperStream - buffer.Length)
                 {
-                    buffer = buffer.Slice(0, (int)(_endInSuperStream - _positionInSuperStream));
+                    buffer = buffer.Slice(0, (int)Math.Max(0L, _endInSuperStream - _positionInSuperStream));
                 }
 
                 int ret = await _superStream.ReadAsync(buffer, cancellationToken).ConfigureAwait(false);

src/libraries/System.IO.Compression/tests/ZipArchive/zip_ReadTests.cs

@@ -889,5 +889,35 @@ public static async Task CompressionMethod_Deflate64_ReturnsDeflate64(bool async
             Assert.Equal(ZipCompressionMethod.Deflate64, readEntry.CompressionMethod);
             await DisposeZipArchive(async, readArchive);
         }
+
+        [Theory]
+        [MemberData(nameof(Get_Booleans_Data))]
+        public static async Task ReadAfterSeekingPastEnd_ReturnsZeroBytes(bool async)
+        {
+            using var ms = new MemoryStream();
+            using (var archive = new ZipArchive(ms, ZipArchiveMode.Create, leaveOpen: true))
+            {
+                var entry = archive.CreateEntry("test.txt", CompressionLevel.NoCompression);
+                using var stream = entry.Open();
+                stream.Write("Hello, World!"u8);
+            }
+
+            ms.Position = 0;
+            using var readArchive = await CreateZipArchive(async, ms, ZipArchiveMode.Read);
+            Stream readStream = await OpenEntryStream(async, readArchive.Entries[0]);
+
+            readStream.Seek(1, SeekOrigin.End);
+            Assert.Equal(14, readStream.Position);
+
+            byte[] buffer = new byte[1024];
+            int bytesRead = async
+                ? await readStream.ReadAsync(buffer)
+                : readStream.Read(buffer, 0, buffer.Length);
+
+            Assert.Equal(0, bytesRead);
+            Assert.Equal(14, readStream.Position);
+
+            await DisposeStream(async, readStream);
+        }
     }
 }