bin: copy: use NGROUPS_MAX when probing user groups

Avoid a theoretical race between getting the number of groups a user
belongs to and actually processing them. We should not need to check
the return value of getgrouplist() as we know the number of groups fit
inside the buffer.

Signed-off-by: Richard Alpe <richard@bit42.se>

src/bin/copy.c

@@ -114,8 +114,7 @@ static gid_t in_group(const char *user, const char *fn, gid_t *gid)
 	if (stat(dir, &st))
 		return 0;
 
-	getgrouplist(user, pw->pw_gid, NULL, &num);
-
+	num = NGROUPS_MAX;
 	groups = malloc(num * sizeof(gid_t));
 	if (!groups) {
 		perror("in_group() malloc");