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Nla Refactor: Blend functions explicit Div0 check

It's an explicit check to prevent division by zero if caller hasn't
done the check. Future patch {D8867} will not use the nla remap
function and thus not do the check. This patch also replaces some
float (==) equality checks with IS_EQF().

Split from {D9247}

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D9694