squashfs: fix off by one in directory counting

This was causing "corrupted directory, had 257 entries instead of max
256" errors, however this was because parseDirectoryHeader had already
added one to the count.

filesystem/squashfs/directory.go

@@ -64,8 +64,8 @@ func parseDirectory(b []byte) (*directory, error) {
 		if err != nil {
 			return nil, fmt.Errorf("could not parse directory header: %v", err)
 		}
-		if directoryHeader.count+1 > maxDirEntries {
-			return nil, fmt.Errorf("corrupted directory, had %d entries instead of max %d", directoryHeader.count+1, maxDirEntries)
+		if directoryHeader.count > maxDirEntries {
+			return nil, fmt.Errorf("corrupted directory, had %d entries instead of max %d", directoryHeader.count, maxDirEntries)
 		}
 		pos += dirHeaderSize
 		for count := uint32(0); count < directoryHeader.count; count++ {