lavf,lavc: free avoptions in a generic way.

It's simpler and less error-prone.

Fixes some memleaks along the way.

libavdevice/vfwcap.c

@@ -234,9 +234,6 @@ static int vfw_read_close(AVFormatContext *s)
         pktl = next;
     }
 
-    av_freep(&ctx->video_size);
-    av_freep(&ctx->framerate);
-
     return 0;
 }