vpxenc: prevent wraparound in the --rate-hist ringbuffer

For clips that are near 60fps and have a lot of alt refs, it's possible
that the ring buffer holding the previous frames sizes/pts could wrap
around, leading to a division by zero.

In addition to checking for this condition in the ring buffer loop,
the buffer size is made dependent on the actual frame rate in use,
rather than defaulting to 60, which should improve accuracy at frame
rates >= ~60.

Change-Id: If5a04d6e847316dc5f7504f25c01164cf9332be8