avfilter/lavfutils: Don't use uninitialized pointers for freeing

Happened on several error conditions, e.g. if there is just no decoder
for the format (like with svg images).

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavfilter/lavfutils.c

@@ -28,9 +28,9 @@ int ff_load_image(uint8_t *data[4], int linesize[4],
     AVInputFormat *iformat = NULL;
     AVFormatContext *format_ctx = NULL;
     AVCodec *codec;
-    AVCodecContext *codec_ctx;
+    AVCodecContext *codec_ctx = NULL;
     AVCodecParameters *par;
-    AVFrame *frame;
+    AVFrame *frame = NULL;
     int frame_decoded, ret = 0;
     AVPacket pkt;
     AVDictionary *opt=NULL;