Propagate allocation failure from ogg_sync_buffer. Instead of segfault, report OP_EFAULT if ogg_sync_buffer returns a null pointer. This allows more graceful recovery by the caller in the unlikely event of a fallible ogg_malloc call.
We do check the return value elsewhere in the code, so the new checks make the code more consistent.
Thanks to https://github.com/xiph/opusfile/issues/36 for reporting.
