Fix the counting of supplementary groups on AIX.

We should not assume that basegid will be present in the list of
gids returned by getgrset().

lib/util/getgrouplist.c

@@ -144,6 +144,7 @@ sudo_getgrouplist2_v1(const char *name, GETGROUPS_T basegid,
 	/* Dynamically-sized group vector, count groups and alloc. */
 	grpsize = 1;	/* reserve one for basegid */
 	if (*grset != '\0') {
+	    grpsize++;	/* at least one supplementary group */
 	    for (cp = grset; *cp != '\0'; cp++) {
 		if (*cp == ',')
 		    grpsize++;