Fix opusenc buffer overflow, channel integer overflow, and division
by zero. (Same code as vorbis-tools oggenc.)

PR: 202941
Obtained from: https://trac.xiph.org/ticket/2212
Obtained from: https://trac.xiph.org/changeset/19117
Obtained from: Fedora vorbis-tools Git (commit 63a1a62d)
Security: CVE-2015-6749
Security: CVE-2014-9638
Security: CVE-2014-9639
Security: a35f415d-572a-11e5-b0a4-f8b156b6dcc8
MFH: 2015Q3