den is a tiny non-zero value in an 80-bit register. It gets spilled to a 64-bit slot on the stack, rounding it to zero, while another copy of it remains in an 80-bit register. The 80-bit version is compared against zero, and is non-zero. The 64-bit version, which is zero, gets used for the division.
It is fully standards-compliant for a C compiler to do this. Some languages may specify otherwise, but often the behavior is unspecified or is implicitly the same as C.
However if you do use floating point, then the kind of dangers you point out make my point even stronger. You could conceivably embed the `calc_div` function in a larger system that knew about pre-validating for div0. But if you want to deal with all possible sources of FP weirdness when doing division, then you really need to concentrate it in the "division expert": i.e. have calc_div pre validate all that stuff, and have its caller accept that errors are a normal result.
If you take the latter approach, then the caller has to have a way of accepting the error gracefully, as a normal thing that might happen. And thus we have a div0 that is an error rather than an exception.