services: mpd: Fix log to file.

(match value (%unset-value ...)) is equivalent here to
(match value (_ ...)). Even if you set 'log-file to some path, it's always
"syslog" in the configuration file.

* gnu/services/audio.scm (mpd): Fix buggy 'match'.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Change-Id: If397919c2844d856c69fe00b8907b7b3fd86e564

gnu/services/audio.scm

@@ -251,16 +251,14 @@ user-group instead~%"))
          (configuration-field-error #f 'group value))))
 
 (define (mpd-log-file-sanitizer value)
-  (match value
-    (%unset-value
   ;; XXX: While leaving the 'sys_log' option out of the mpd.conf file is
   ;; supposed to cause logging to happen via systemd (elogind provides a
   ;; compatible interface), this doesn't work (nothing gets logged); use
   ;; syslog instead.
-     "syslog")
-    ((? string?)
-     value)
-    (_ (configuration-field-error #f 'log-file value))))
+  (let ((value (maybe-value value "syslog")))
+    (if (string? value)
+        value
+        (configuration-field-error #f 'log-file value))))
 
 ;;;