smooth: don't adjust invalid time of last update

smooth.c

@@ -246,6 +246,7 @@ handle_slew(struct timespec *raw, struct timespec *cooked, double dfreq,
       update_smoothing(cooked, doffset, dfreq);
   }
 
+  if (!UTI_IsZeroTimespec(&last_update))
     UTI_AdjustTimespec(&last_update, cooked, &last_update, &delta, dfreq, doffset);
 }
 
@@ -264,6 +265,8 @@ void SMT_Initialise(void)
   max_freq *= 1e-6;
   max_wander *= 1e-6;
 
+  UTI_ZeroTimespec(&last_update);
+
   LCL_AddParameterChangeHandler(handle_slew, NULL);
 }