libnm: fix endless loop in nm_vpn_service_plugin_read_vpn_details()

Previously, a "DONE\n" was required to break the loop.
4 jobs for th/read-vpn-plugins-tests in 28 minutes and 58 seconds (queued for 48 minutes and 11 seconds)
Status Job ID Name Coverage
  Test
passed #32947
allowed to fail
checkpatch

00:02:43

passed #32948
f28_build_default

00:14:22

passed #32949
f28_rpm_autotools_dbg

00:13:47

passed #32950
f28_rpm_meson_dbg

00:11:50