fix endianness problem with the tail of the signature comment

The two 0xff bytes were intended to easily distinguish files with
whole file signatures from those without, but I got the endianness
backwards.  Go ahead and fix that, as long as I'm making changes to
the verifier anyway.

Check for a signature that includes the sequence 0x50 0x4b 0x05 0x06,
which looks to minzip like the start of the EOCD block.