math: fix pow signed shift ub

j is int32_t and thus j<<31 is undefined if j==1, so j is changed to
uint32_t locally as a quick fix, the generated code is not affected.

(this is a strict conformance fix, future c standard may allow 1<<31,
see DR 463.  the bug was inherited from freebsd fdlibm, the proper fix
is to use uint32_t for all bit hacks, but that requires more intrusive
changes.)

reported by Daniel Sabogal

src/math/pow.c

@@ -125,11 +125,11 @@ double pow(double x, double y)
 		else if (iy >= 0x3ff00000) {
 			k = (iy>>20) - 0x3ff;  /* exponent */
 			if (k > 20) {
-				j = ly>>(52-k);
+				uint32_t j = ly>>(52-k);
 				if ((j<<(52-k)) == ly)
 					yisint = 2 - (j&1);
 			} else if (ly == 0) {
-				j = iy>>(20-k);
+				uint32_t j = iy>>(20-k);
 				if ((j<<(20-k)) == iy)
 					yisint = 2 - (j&1);
 			}