fix offset check

Author: dellis1972

LibZipSharp/Xamarin.Tools.Zip/ZipArchive.cs

@@ -790,8 +790,8 @@ internal static unsafe Int64 stream_callback (IntPtr state, IntPtr data, UInt64
 				case SourceCommand.Seek:
 					Native.zip_error_t error;
 					Int64 offset = Native.zip_source_seek_compute_offset ((UInt64)stream.Position, (UInt64)stream.Length, data, len, out error);
-					if (offset == -1)
-						return -1;
+					if (offset < 0)
+						return offset;
 					stream.Seek ((long)offset, SeekOrigin.Begin);
 					break;