decoder: reset preroll on flush

We don´t want to discard decoded data if seeking during prefoll then
the timestamps restart from lower values.

src/input/decoder.c

@@ -1396,6 +1396,10 @@ static void DecoderProcessFlush( decoder_t *p_dec )
                 vlc_object_release( p_vout );
         }
     }
+
+    vlc_mutex_lock( &p_owner->lock );
+    p_owner->i_preroll_end = INT64_MIN;
+    vlc_mutex_unlock( &p_owner->lock );
 }
 
 /**