avcodec/avcodec: Simplify accessing AVSubtitleRect via dedicated pointer

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/avcodec.c

@@ -453,12 +453,15 @@ void avsubtitle_free(AVSubtitle *sub)
     int i;
 
     for (i = 0; i < sub->num_rects; i++) {
-        av_freep(&sub->rects[i]->data[0]);
-        av_freep(&sub->rects[i]->data[1]);
-        av_freep(&sub->rects[i]->data[2]);
-        av_freep(&sub->rects[i]->data[3]);
-        av_freep(&sub->rects[i]->text);
-        av_freep(&sub->rects[i]->ass);
+        AVSubtitleRect *const rect = sub->rects[i];
+
+        av_freep(&rect->data[0]);
+        av_freep(&rect->data[1]);
+        av_freep(&rect->data[2]);
+        av_freep(&rect->data[3]);
+        av_freep(&rect->text);
+        av_freep(&rect->ass);
+
         av_freep(&sub->rects[i]);
     }