avformat/qoadec: Check ffio_ensure_seekback()

Fixes Coverity issue #1598406.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/qoadec.c

@@ -41,6 +41,7 @@ static int qoa_read_header(AVFormatContext *s)
 {
     AVIOContext *pb = s->pb;
     AVStream *st;
+    int ret;
 
     st = avformat_new_stream(s, NULL);
     if (!st)
@@ -52,7 +53,9 @@ static int qoa_read_header(AVFormatContext *s)
     st->duration = avio_rb32(pb);
     st->start_time = 0;
 
-    ffio_ensure_seekback(pb, 4);
+    ret = ffio_ensure_seekback(pb, 4);
+    if (ret < 0)
+        return ret;
     st->codecpar->ch_layout.nb_channels = avio_r8(pb);
     if (st->codecpar->ch_layout.nb_channels == 0)
         return AVERROR_INVALIDDATA;