Fix some things that I was mistakenly testing with an old compiler.

Fix some things that I was mistakenly testing with an old compiler.

sqrt should not trap on qNaN, which requires some care with
comparisons.  Further, since sqrt(-0) is supposed to be -0, we can't
just use flo:safe-negative? (which returns true for -0.); we must
instead use (flo:safe< x 0.) (which returns false for -0.).