complex: fix ctanh(+-0+i*nan) and ctanh(+-0+-i*inf)

These cases were incorrect in C11 as described by
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1886.htm

src/complex/ctanh.c

@@ -101,11 +101,13 @@ double complex ctanh(double complex z)
 	}
 
 	/*
+	 * ctanh(+-0 + i NAN) = +-0 + i NaN
+	 * ctanh(+-0 +- i Inf) = +-0 + i NaN
 	 * ctanh(x + i NAN) = NaN + i NaN
 	 * ctanh(x +- i Inf) = NaN + i NaN
 	 */
 	if (!isfinite(y))
-		return CMPLX(y - y, y - y);
+		return CMPLX(x ? y - y : x, y - y);
 
 	/*
 	 * ctanh(+-huge + i +-y) ~= +-1 +- i 2sin(2y)/exp(2x), using the

src/complex/ctanhf.c

@@ -50,7 +50,7 @@ float complex ctanhf(float complex z)
 	}
 
 	if (!isfinite(y))
-		return CMPLXF(y - y, y - y);
+		return CMPLXF(ix ? y - y : x, y - y);
 
 	if (ix >= 0x41300000) { /* x >= 11 */
 		float exp_mx = expf(-fabsf(x));