avfilter/select: initialize prev_selected_n to NAN

As per the doc, prev_selected_n should be NAN at the start.
However, this was never set.

libavfilter/f_select.c

@@ -233,6 +233,7 @@ static int config_input(AVFilterLink *inlink)
 
     select->var_values[VAR_TB] = av_q2d(inlink->time_base);
 
+    select->var_values[VAR_PREV_SELECTED_N]   = NAN;
     select->var_values[VAR_PREV_PTS]          = NAN;
     select->var_values[VAR_PREV_SELECTED_PTS] = NAN;
     select->var_values[VAR_PREV_SELECTED_T]   = NAN;