[xad] Distingush RARSFX with ZIP signature from ZIP Archive with RAR signature

[PaulTaykalo]

Somewhat theoritical case happens when unar tries to unarchive sfx archives.
SFX formats have basically the logic when first part of the file is skipped until signature found.
some SFX archives can have signatures from other archives
i.e.
sfx rar with .zip files inside
sfx zip with .rar files inside

unar need to handle these cases in a correct way

[PaulTaykalo]

Fixed. by checking which signature. comes first