Print unknown_str in case of error

func can return NULL in case of error, but no checking is done when
printing, and unknown_str wasn't being used anywhere.

slstatus.c

@@ -87,8 +87,10 @@ main(int argc, char *argv[])
 
 		status[0] = '\0';
 		for (i = len = 0; i < LEN(args); i++) {
+			const char * res = args[i].func(args[i].args);
+			res = (res == NULL) ? unknown_str : res;
 			len += snprintf(status + len, sizeof(status) - len,
-			                args[i].fmt, args[i].func(args[i].args));
+			                args[i].fmt, res);
 
 			if (len >= sizeof(status)) {
 				status[sizeof(status) - 1] = '\0';