file scan hangs on files that don't have read permissions

[richardlehane]

(reported by Greg Lepore):
I've also notice that Siegfried has an issue when attempting to scan a file that it doesn't have read permissions on. sf just hangs:

desktop:/Downloads/formats/test/temp$ chmod 333 viewport.bmp
desktop:/Downloads/formats/test/temp$ sf viewport.bmp
[FILE] //Downloads/formats/test/temp/viewport.bmp
[ERROR] open viewport.bmp: permission denied

Have to kill the process manually. The same thing happens with -coe.

[fozboz]

Confirmed in server mode too.