lavfi: fix null checking in avfilter_config_links()

Move the first use of `link' after the null pointer check.
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/avfilter.c

@@ -222,11 +222,11 @@ int avfilter_config_links(AVFilterContext *filter)
 
     for (i = 0; i < filter->nb_inputs; i ++) {
         AVFilterLink *link = filter->inputs[i];
-        AVFilterLink *inlink = link->src->nb_inputs ?
-            link->src->inputs[0] : NULL;
+        AVFilterLink *inlink;
 
         if (!link) continue;
 
+        inlink = link->src->nb_inputs ? link->src->inputs[0] : NULL;
         link->current_pts = AV_NOPTS_VALUE;
 
         switch (link->init_state) {