input_clock: don't update invalid points on rate change

Only call UpdateListener() if the clock is valid.

This fixes the following assert:

vlc: ../../src/clock/clock.c:585: vlc_clock_monotonic_to_system: Assertion `ctx->wait_sync_ref.stream != VLC_TICK_INVALID' failed.

This was occurring after a pause, a flush and a resume (that triggers a
rate change).

The assertion is new, but VLC_TICK_INVALID was used as a reference point
before in that case.

src/clock/input_clock.c

@@ -348,8 +348,9 @@ void input_clock_ChangeRate( input_clock_t *cl, float rate )
          * from the start */
         cl->ref.system = cl->last.system
             - (vlc_tick_t) ((cl->last.system - cl->ref.system) / rate * oldrate);
+
+        UpdateListener( cl, false );
     }
-    UpdateListener( cl, false );
 }
 
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