avfilter/vf_pad: Fix segfault if reconfiguration fails

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_pad.c

@@ -210,11 +210,15 @@ static int config_output(AVFilterLink *outlink)
 static AVFrame *get_video_buffer(AVFilterLink *inlink, int w, int h)
 {
     PadContext *s = inlink->dst->priv;
+    AVFrame *frame;
+    int plane;
+
+    if (s->inlink_w <= 0)
+        return NULL;
 
-    AVFrame *frame = ff_get_video_buffer(inlink->dst->outputs[0],
+    frame = ff_get_video_buffer(inlink->dst->outputs[0],
                                 w + (s->w - s->in_w),
                                 h + (s->h - s->in_h) + (s->x > 0));
-    int plane;
 
     if (!frame)
         return NULL;