rrcache: don't clobber pkt if failing the second step

I hope fixing this bug should diminish the recent experiences of Google
domains failing to resolve on Turris Omnia.
3 jobs for rrcache-pkt-clobber in 1 minute and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #8213
docker linux amd64
build:linux:amd64

00:00:47

 
  Test
passed #8215
docker linux amd64
deckard:linux:amd64

00:00:31

passed #8214
docker linux amd64
test:linux:amd64

00:00:21