avidec: fix "avidec.c:362: warning: st may be used uninitialized in this function"

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avidec.c

@@ -699,6 +699,7 @@ static int avi_read_header(AVFormatContext *s, AVFormatParameters *ap)
             }
             break;
         case MKTAG('s', 't', 'r', 'd'):
+            st = s->streams[stream_index];
             if (stream_index >= (unsigned)s->nb_streams || st->codec->extradata_size) {
                 avio_skip(pb, size);
             } else {