afh: Limit chunk numbers to 32 bit.

afh: Limit chunk numbers to 32 bit.

The number of chunks and the chunk offsets are stored in the audio
file table as 32 bit unsigned integers. Thus, chunk numbers and
sizes cannot exceed 2^32 - 1. Make this fact obvious by changing
the corresponding parameters of aac_afh_get_chunk() from size_t or
unsigned long to uint32_t.