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.

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>
Signed-off-by: Mark Harris <mark.hsj@gmail.com>