Backend: fixup now that mount_apex direct .img mount works

META-INF/com/google/android/update-binary

@@ -109,7 +109,7 @@ umount_apex() {
   for dest in $(find /apex -type d -mindepth 1 -maxdepth 1); do
     loop=$(mount | grep $dest | grep loop | cut -d\  -f1);
     umount -l $dest;
-    [ "$loop" ] && losetup -d $loop;
+    losetup $loop >/dev/null 2>&1 && losetup -d $loop;
   done;
   [ -f /apex/apextmp ] && umount /apex;
   rm -rf /apex/apexak3 /apex 2>/dev/null;