Check the returned audio_buf_info fields

At least on one system zero is returned in either fragsize or
fragstotal (reported by Dave Scott), this results in an audio_calloc
failing the audio_bug check and another ominous error message. Fail
early and blame the system.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4699 c046a42c-6fe2-441c-8c8c-71466251a162
