Fix Modf leaving a reused Infinity or NaN unchanged

[SashaMIT]

Summary

Modf wrote the coefficient, exponent, and sign of the integral and fractional parts and left Form untouched. A destination that was Infinity or NaN still stringified as that form.

Modf of 1.5 into an Infinity and a NaN returned Infinity and NaN. It now returns 1 and 0.5. A value smaller than 1, such as 0.00001, now has integral part 0 instead of Infinity. An integer such as 20 now has fractional part 0 instead of NaN.

Test plan

• TestModfResetsNonFiniteDestination failed on tip (integ = Infinity) and passes with the change
• go test -count=1 . passed