ffserver: do not ignore lseek() return

Should fix FFmpeg Coverity Scan issue #732177
Signed-off-by: Reynaldo H. Verdejo Pinochet <r.verdejo@sisa.samsung.com>

ffserver.c

@@ -2755,8 +2755,11 @@ static int http_receive_data(HTTPContext *c)
         /* a packet has been received : write it in the store, except
            if header */
         if (c->data_count > FFM_PACKET_SIZE) {
-            /* XXX: use llseek or url_seek */
-            lseek(c->feed_fd, feed->feed_write_index, SEEK_SET);
+            /* XXX: use llseek or url_seek
+             * XXX: Should probably fail? */
+            if (lseek(c->feed_fd, feed->feed_write_index, SEEK_SET) == -1)
+                http_log("Seek to %"PRId64" failed\n", feed->feed_write_index);
+
             if (write(c->feed_fd, c->buffer, FFM_PACKET_SIZE) < 0) {
                 http_log("Error writing to feed file: %s\n", strerror(errno));
                 goto fail;