sysctl: Correct stat on /etc/sysctl.conf

sysctl --system fails when the file /etc/sysctl.conf doesn't
exists. This happens due to wrong check of stat(2) return code.

Reference:
  https://www.freelists.org/post/procps/sysctlSigned-off-by: Craig Small <csmall@enc.com.au>

sysctl.c

@@ -655,7 +655,7 @@ static int PreloadSystem(void)
 	}
 
 
-	if (stat(DEFAULT_PRELOAD, &ts) < 0 || S_ISREG(ts.st_mode)) {
+	if (stat(DEFAULT_PRELOAD, &ts) == 0 && S_ISREG(ts.st_mode)) {
 		if (!Quiet)
 			printf(_("* Applying %s ...\n"), DEFAULT_PRELOAD);
 		rc |= Preload(DEFAULT_PRELOAD);