src/newusers.c: A user/group name with a leading digit is valid

Only consider a string to be a UID/GID if it is all digits.

Here's a reproducer of the bug:

	$ echo 'foo:p::1a::/tmp/nonexistent:/usr/bin/false' > x
	$ sudo newusers ./x
	newusers: invalid group ID '1a'
	newusers: line 1: can't create group

Where the expected behavior would be the same as for a group name that
doesn't start with a digit:

	$ echo 'foo:p::a1a::/tmp/nonexistent:/usr/bin/false' > x
	$ sudo newusers ./x
	$ tail -n1 /etc/group
	a1a:x:1004:
	$ tail -n1 /etc/passwd
	foo:x:1004:1004::/tmp/nonexistent:/usr/bin/false

Closes: <#1474>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

Author: alejandro-colomar

src/newusers.c

@@ -56,6 +56,7 @@
 #include "shadow/gshadow/sgrp.h"
 #include "shadowlog.h"
 #include "sssd.h"
+#include "string/ctype/strisascii/strisdigit.h"
 #include "string/sprintf/snprintf.h"
 #include "string/strcmp/streq.h"
 #include "string/strdup/strdup.h"
@@ -248,7 +249,7 @@ static int add_group (const char *name, const char *gid, gid_t *ngid, uid_t uid)
 		return 0;
 	}
 
-	if (isdigit (gid[0])) {
+	if (!streq(gid, "") && strisdigit(gid)) {
 		/*
 		 * The GID is a number, which means either this is a brand
 		 * new group, or an existing group.
@@ -292,7 +293,7 @@ static int add_group (const char *name, const char *gid, gid_t *ngid, uid_t uid)
 	/*
 	 * Now I have all of the fields required to create the new group.
 	 */
-	if (!streq(gid, "") && (!isdigit(gid[0]))) {
+	if (!streq(gid, "") && !strisdigit(gid)) {
 		grent.gr_name = xstrdup (gid);
 	} else {
 		grent.gr_name = xstrdup (name);
@@ -357,7 +358,7 @@ static int get_user_id (const char *uid, uid_t *nuid) {
 	 * The first guess for the UID is either the numerical UID that the
 	 * caller provided, or the next available UID.
 	 */
-	if (isdigit (uid[0])) {
+	if (!streq(uid, "") && strisdigit(uid)) {
 		if ((get_uid(uid, nuid) == -1) || (*nuid == (uid_t)-1)) {
 			fprintf (stderr,
 			         _("%s: invalid user ID '%s'\n"),