math: fix expf(-NAN) and exp2f(-NAN) to return -NAN instead of 0

expf(-NAN) was treated as expf(-large) which unconditionally
returns +0, so special case +-NAN.
reported by Petr Hosek.

src/math/exp2f.c

@@ -91,6 +91,8 @@ float exp2f(float x)
 	/* Filter out exceptional cases. */
 	ix = u.i & 0x7fffffff;
 	if (ix > 0x42fc0000) {  /* |x| > 126 */
+		if (ix > 0x7f800000) /* NaN */
+			return x;
 		if (u.i >= 0x43000000 && u.i < 0x80000000) {  /* x >= 128 */
 			x *= 0x1p127f;
 			return x;

src/math/expf.c

@@ -39,6 +39,8 @@ float expf(float x)
 
 	/* special cases */
 	if (hx >= 0x42aeac50) {  /* if |x| >= -87.33655f or NaN */
+		if (hx > 0x7f800000) /* NaN */
+			return x;
 		if (hx >= 0x42b17218 && !sign) {  /* x >= 88.722839f */
 			/* overflow */
 			x *= 0x1p127f;