reference: don't report synchronized status after unknown step

The source handler resets SST instances on an unknown step, which
makes the sources unselectable, but SRC_SelectSource() doesn't call
REF_SetUnsynchronised() when no source is selectable.

Handle the step in the reference handler.

Fixes: 049eae661a ("sources: keep synchronized status with unreachable/unselectable sources")

sources.c

@@ -1128,7 +1128,7 @@ slew_sources(struct timespec *raw, struct timespec *cooked, double dfreq,
   }
 
   if (change_type == LCL_ChangeUnknownStep) {
-    /* After resetting no source is selectable, set reference unsynchronised */
+    /* Update selection status */
     SRC_SelectSource(NULL);
   }
 }