ogg_afh_common.c: Check return value of ogg_page_granulepos().

ogg_afh_common.c: Check return value of ogg_page_granulepos().

This function may return -1, and we do not want to use a negative
granule position for the computation of the duration of the ogg file.

Note that oac_get_file_info() loops over all ogg pages twice, but
the second loop does not have the same problem as we already check
the return value there.