cmd/web: fix wrong ExternalURL when specify port via CLI flag
cmd/web: fix wrong ExternalURL when specify port via CLI flag

Fixes #5936.

chriswatt/gogs,gogits/gogs,mukaiu/gogs,chriswatt/gogs,mukaiu/gogs,gogits/gogs,chriswatt/gogs,mukaiu/gogs,gogits/gogs