Merge pull request #196 from fgimian/fix-extraction-of-sfx-archives

Use a wider check position so SFX archives are correctly detected.

SevenZip/SevenZipExtractor.cs

@@ -454,7 +454,7 @@ private IInStream GetArchiveStream(bool dispose)
         /// <returns>OperationResult.Ok if Open() succeeds.</returns>
         private OperationResult OpenArchiveInner(IInStream archiveStream, IArchiveOpenCallback openCallback)
         {
-            ulong checkPos = 1 << 15;
+            ulong checkPos = 1 << 23;
             var res = _archive.Open(archiveStream, ref checkPos, openCallback);
 
             return (OperationResult)res;