dak/check_archive.py: multi-archive adjustment for check_checksums()

Locations went away with the multi-archive changes, but
check_checksums() was still using them.
Reported-by: NMatthias Klumpp <mak@debian.org>

dak/check_archive.py

@@ -274,7 +274,7 @@ def check_checksums():
 
     print "Checking file checksums & sizes..."
     for f in q:
-        filename = os.path.abspath(os.path.join(f.location.path, f.filename))
+        filename = f.fullpath
 
         try:
             fi = utils.open_file(filename)