audio/flac: Fix out-of-bounds read

Obtained from: https://github.com/xiph/flac/commit/2e7931c27eb15e387da440a37f12437e35b22dd4
Security: 49346de2-b015-11eb-9bdf-f8b156b6dcc8
Security: CVE-2020-0499

(cherry picked from commit 3984e6561a7ee7383c502456ae27ce48ee28a028)