Align severity for CVE-2018-19432 with CVE-2018-13139

They have the same underlying issue, the -- missing channel number check
in sndfile-deinterleave program, and not a problem in the library.

Both are treated as different vulnerabilities by the MITRE CNA but have
the same fix as commited upstream via

https://github.com/erikd/libsndfile/commit/aaea680337267bfb6d2544da878890ee7f1c5077

Track both issues in the same way and demote severity of CVE-2018-19432
to unimportant.