Fix encoder crash with non-positive sample rates.

Fix encoder crash with non-positive sample rates.

Input files with sampling rate 0 are useless and can make
libvorbis access invalid memory because the logic in
_vp_psy_init (and probably other functions) isn't prepared for
it. A sanity check lets the library refuse those inputs
gracefully in the initialization functions before they can
do harm.

Patch by Martin Steghöfer <martin@steghoefer.eu>

Fixes #2078
Bug-Debian: https://bugs.debian.org/716613

svn path=/trunk/vorbis/; revision=19422