avcodec: fix uninitialized variable read

This cna happen if the user tries to call the new decode API for
subtitles.

Fixes CID 1402071.

(cherry picked from commit b4b8ca24f62473528949fe047085eb084364124b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/utils.c

@@ -2719,7 +2719,7 @@ void avsubtitle_free(AVSubtitle *sub)
 
 static int do_decode(AVCodecContext *avctx, AVPacket *pkt)
 {
-    int got_frame;
+    int got_frame = 0;
     int ret;
 
     av_assert0(!avctx->internal->buffer_frame->buf[0]);