Handle negative numbers in sqrt properly.

Closes #9987.

src/libcore/num/mod.rs

@@ -1501,6 +1501,8 @@ pub trait Float: Signed + Primitive {
     fn frac_1_sqrt2() -> Self;
 
     /// Take the square root of a number.
+    ///
+    /// Returns NaN if `self` is not a non-negative number.
     fn sqrt(self) -> Self;
     /// Take the reciprocal (inverse) square root of a number, `1/sqrt(x)`.
     fn rsqrt(self) -> Self;