Fix NULL check in opus_tags_add_comment().

In 0221ca95 the allocation result went from being stored
 directly in "_tags->user_comments[ncomments]" to being stored in
 the temporary "comment".
However, the NULL check for allocation failure was not updated to
 match.
This meant this function would almost always fail, unless you had
 added binary metadata first.

Fixes Coverity CID 149874.