makebuildserver: delete cached files with zero size

If download fails to connect, it'll make a zero size file.  Then the next
run on ./makebuildserver will fail because the checksum will not be right.
2 jobs for makebuildserver-for-debian-jenkins
Status Job ID Name Coverage
  Test
passed #593452
test

00:30:00

passed #593446
test

00:29:37