dgit: avoid "Use of uninitialized value $got in concatenation"

When dgit wanted to report that a file it downloaded had the wrong
checksum, it would instead crash with this message.  This was due to
complete_file_from_dsc's $got (which is relied on by the callers of
$checkhash) being shadowed inside $checkhash.

This is part of #867185/#867185.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>