Input files with sampling rate 0 are useless and can make oggenc crash
because neither oggenc itself nor libvorbis are prepared for this case.
A sanity check lets the program refuse those inputs gracefully without crash.

Patch by Martin Steghöfer <martin@steghoefer.eu>

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

svn path=/trunk/vorbis-tools/; revision=19423