vss: Don't prefault header.

Commit 7bba6232 (vss: Mmap audio files using MAP_POPULATE.) introduced
read-ahead for chunks of the mmapped audio file. However, it missed
the fact that for ogg streams chunk 0 is created on the fly and stored
in a dynamically allocated buffer. Read-ahead on this buffer is likely
to access memory not owned by the process and might lead to a segfault.

Fix this bug by not performing read-ahead for chunk zero.