in clock_getres, check for null pointer before storing result

POSIX allows a null pointer, in which case the function only checks
the validity of the clock id argument.

src/time/clock_getres.c

@@ -8,7 +8,7 @@ int clock_getres(clockid_t clk, struct timespec *ts)
 	if (SYS_clock_getres != SYS_clock_getres_time64) {
 		long ts32[2];
 		int r = __syscall(SYS_clock_getres, clk, ts32);
-		if (!r) {
+		if (!r && ts) {
 			ts->tv_sec = ts32[0];
 			ts->tv_nsec = ts32[1];
 		}