lib/sgetgrent.c: fix null pointer dereference

If reallocation fails in function list, then reset the size to 0 again.
Without the reset, the next call assumes that `members` points to
a memory location with reserved space.

Also use size_t instead of int for size to prevent signed integer
overflows. The length of group lines is not limited.

Fixes 45c0003e53 (4.14 release series)

Proof of Concept:

-  Prepare a group file (one long group line and a shorter one, both with a list of users)

	$ echo -n "root:x:0:" > /tmp/uwu
	$ yes , | tr -d '\n' | dd of=/tmp/uwu bs=10 count=3145728 seek=1 conv=notrunc iflag=fullblock
	$ echo -e "\nbin:x:1:," >> /tmp/uwu

-  Run grpck with tight memory constraints

	$ ulimit -d 102400
	$ grpck /tmp/uwu
	Segmentation fault (core dumped)

Reviewed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Samanta Navarro <ferivoz@riseup.net>
Cherry-picked-from: a9e07c0feb ("lib/sgetgrent.c: fix null pointer dereference")
Link: <https://github.com/shadow-maint/shadow/pull/904>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/sgetgrent.c

@@ -37,8 +37,8 @@
 static char **list (char *s)
 {
 	static char **members = NULL;
-	static int size = 0;	/* max members + 1 */
-	int i;
+	static size_t size = 0;	/* max members + 1 */
+	size_t i;
 
 	i = 0;
 	for (;;) {
@@ -47,8 +47,10 @@ static char **list (char *s)
 		if (i >= size) {
 			size = i + 100;	/* at least: i + 1 */
 			members = REALLOCF(members, size, char *);
-			if (!members)
+			if (!members) {
+				size = 0;
 				return NULL;
+			}
 		}
 		if (!s || s[0] == '\0')
 			break;