Fix misplaced cast that caused an overflow and incorrect rate adaptation
behavior for high data rates. This in particular will have affected 4k encodes
but could also have come into play for some higher rate 1080p cases.

In our standard test sets the quality impact is small though several high rate
clips show improved rate accuracy. This can also impact the number of recode
loop hits and on one problem 4k  clip the encode time for speeds 0 and 1 was
reduced by >25%

Change-Id: I108da7ca42f3bc95c5825dd33c9d84583227dac1