Prevent out of boundary access

If lines start with '\0' then it is possible to trigger out of
boundary accesses.

Check if indices are valid before accessing them.

Signed-off-by: Samanta Navarro <ferivoz@riseup.net>

src/suauth.c

@@ -68,8 +68,9 @@ int check_su_auth (const char *actual_id,
 
 	while (fgets (temp, sizeof (temp), authfile_fd) != NULL) {
 		lines++;
+		endline = strlen(temp) - 1;
 
-		if (temp[endline = strlen (temp) - 1] != '\n') {
+		if (temp[0] == '\0' || temp[endline] != '\n') {
 			SYSLOG ((LOG_ERR,
 				 "%s, line %d: line too long or missing newline",
 				 SUAUTHFILE, lines));