decode: Initialize ret before using it

libavcodec/decode.c:608:9: warning: variable 'ret' is
      used uninitialized whenever 'if' condition is false

(cherry picked from libav commit efddf2c0)

libavcodec/decode.c

@@ -695,7 +695,7 @@ static int compat_decode(AVCodecContext *avctx, AVFrame *frame,
                          int *got_frame, const AVPacket *pkt)
 {
     AVCodecInternal *avci = avctx->internal;
-    int ret;
+    int ret = 0;
 
     av_assert0(avci->compat_decode_consumed == 0);