Avoid calculating NaN for brush tip state parameters

Base brush attributes and individual brush behavior modifier values are guaranteed to be finite. However, two finite values multiplied together can overflow to infinity, and later multiplying that infinity by zero results in NaN, which is an invalid value for e.g. a brush tip width or height. This is causing occasional fuzz test failures.

If the multiplications had happened in a different order, we would first multiply zero by a finite value to get zero, then multiply that by the other finite value to get zero. In this particular context, infinities can only arise due to overflow, so it seems reasonable in this context to define zero times infinity as zero. Therefore, this change fixes the issue by checking for zeros before multiplying behavior modifiers together.