samiparse: fix handling of self-closing tags

samiparse: fix handling of self-closing tags

We would check the wrong string (rest of line rather than element)
for the / suffix of self-closing tags, which is not only wrong but
also has atrocious performance with certain strings like the garbled
nonsense clusterfuzz feeds us, which might cause discoverer to time
out when processing garbled SAMI files.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=47461

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2685>