libmultipath: don't print error message if WATCHDOG_USEC is 0

WATCHDOG_USEC may be set to 0, which means that the watchdog
is disabled in systemd.

Fixes: 9366cfb ("multipathd: Implement systemd watchdog integration")
Signed-off-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>

libmultipath/config.c

@@ -865,6 +865,9 @@ static void set_max_checkint_from_watchdog(struct config *conf)
 	unsigned long checkint;
 
 	if (envp && sscanf(envp, "%lu", &checkint) == 1) {
+		if (checkint == 0)
+			/* watchdog disabled */
+			return;
 		/* Value is in microseconds */
 		checkint /= 1000000;
 		if (checkint < 1 || checkint > UINT_MAX) {