Fix undefined behavior in f64::sqrt
Fix undefined behavior in f64::sqrt

This fixes a reappearance of bug #9987 introduced in
1ddee8070d3cb83609b1f71c29e3deda3d30fd51, which caused
f64::tests::test_sqrt_domain to fail (at least on some systems).

graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust