Fixes for r17514.

Actually allocate the right number of comments, and add an extra check against
 i+1 overflowing (which could happen with a 4 GB comment packet on a 64-bit
 machine... unlikely, but possible).


git-svn-id: https://svn.xiph.org/trunk/Tremor@17515 0101bb08-14d6-0310-b084-bc0e0c8e3800