v4l2: do not assert on a value received from outside of Libav

libavdevice/v4l2.c

@@ -460,7 +460,12 @@ static int mmap_read_frame(AVFormatContext *ctx, AVPacket *pkt)
 
         return AVERROR(errno);
     }
-    assert (buf.index < s->buffers);
+
+    if (buf.index >= s->buffers) {
+        av_log(ctx, AV_LOG_ERROR, "Invalid buffer index received.\n");
+        return AVERROR(EINVAL);
+    }
+
     if (s->frame_size > 0 && buf.bytesused != s->frame_size) {
         av_log(ctx, AV_LOG_ERROR,
                "The v4l2 frame is %d bytes, but %d bytes are expected\n",