Fix incorrect truncation of floating-point data
If the entire parenthesized division expression is cast to float, as it
was prior to this commit, C performs truncating integer division before
converting to float, which always leads to a result of zero.  `git-blame'
traces the bug to commit d5ed7e75d9ca42938278bc887d44d9a8a634dcdf, which
added the faulty parentheses as a matter of style. No parentheses are
necessary, since increments have higher precedence than type-casts,
type-casts have higher precedence than division, and division has
higher precedence than comparison. The operations will execute in the
desired order.