avformat/aviobuf: Check that avio_seek() target is non negative

Fixes out of array access
Suggested-by: Andrew Scherkus <scherkus@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/aviobuf.c

@@ -223,6 +223,9 @@ int64_t avio_seek(AVIOContext *s, int64_t offset, int whence)
             return offset1;
         offset += offset1;
     }
+    if (offset < 0)
+        return AVERROR(EINVAL);
+
     offset1 = offset - pos;
     if (!s->must_flush && (!s->direct || !s->seek) &&
         offset1 >= 0 && offset1 <= buffer_size) {