BUG/MEDIUM: ring: fix creation of server in uninitialized ring

BUG/MEDIUM: ring: fix creation of server in uninitialized ring

If a "ring" section initialization fails (e.g. due to a duplicate name,
invalid chars, or missing memory), any subsequent "server" statement that
appears in the same section will crash the config parser by dereferencing
the currently NULL cfg_sink. E.g:

    ring x
    ring x                 # fails on "already exists"
       server srv 1.1.1.1  # crashes on cfg_sink==NULL

All other statements have a test for this but "server" was missing it,
so this patch adds it.

Thanks to Joel Hutchinson for reporting this issue.

This must be backported as far as 2.2.