Make sure err never gets masked

Defining err as named return parameter will make sure the variable gets
assigned before returning and thus avoid masking

Docker-DCO-1.1-Signed-off-by: Johannes 'fish' Ziemke <github@freigeist.org> (github: discordianfish)
Merge pull request #7100 from discordianfish/fix-port-allocation

Fix masked err in portmapper