misidentifications when running with -multi flag

[richardlehane]

Testing on Greg Lepore's PRONOM FILES corpus have found that the -multi flag can cause misidentifications for a small number of OLE container files. These misidentifications don't reliably occur so looks to be some kind of race condition

[richardlehane]

Traced this down to the github.com/richardlehane/mscfb library. A global sectorSize variable made that package unsafe for concurrent usage.