avfilter/vf_idet: factorize av_frame_free(&idet->prev)

Found-by: Pascal Massimino <pascal.massimino@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_idet.c

@@ -249,6 +249,8 @@ static int filter_frame(AVFilterLink *link, AVFrame *picref)
         return ff_filter_frame(ctx->outputs[0], picref);
     }
 
+    av_frame_free(&idet->prev);
+
     if(   picref->width  != link->w
        || picref->height != link->h
        || picref->format != link->format) {
@@ -256,13 +258,10 @@ static int filter_frame(AVFilterLink *link, AVFrame *picref)
         link->dst->inputs[0]->w      = picref->width;
         link->dst->inputs[0]->h      = picref->height;
 
-        av_frame_free(&idet->prev);
         av_frame_free(&idet->cur );
         av_frame_free(&idet->next);
     }
 
-    if (idet->prev)
-        av_frame_free(&idet->prev);
     idet->prev = idet->cur;
     idet->cur  = idet->next;
     idet->next = picref;