bug#13297: hi-lock-unface-buffer fails on face lists

[Jonas Bernoulli]

Before commit 111129 hi-lock-unface-buffer could handle face symbols as
well as face lists.  After this commit it fails on lists.

Third-party library highlight-symbol uses hi-lock-set-pattern passing it
a face list.  The above change breaks this usage which isn't disallowed
in hi-lock-set-pattern's doc-string.  As a result this library cannot
remove the highlights it added with the help of hi-lock.

  Jonas