[APFloat] Fix APFloat::getExactInverse when the input is nullptr. This is a...

[APFloat] Fix APFloat::getExactInverse when the input is nullptr. This is a regression introduced by r285105.

Reviewers: kbarton, echristo, iteratee, eugenis

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D26017

llvm-svn: 285256