libcasper: Neuter false positive -Wuse-after-free warnings from GCC 13

GCC 13 incorrectly thinks a call to free after a failed realloc is a
use after free.

lib/libcasper/services/cap_grp/cap_grp.c: In function 'group_resize':
lib/libcasper/services/cap_grp/cap_grp.c:65:17: error: pointer 'buf' may be used after 'realloc' [-Werror=use-after-free]

65 |                 free(buf);
   |                 ^~~~~~~~~

lib/libcasper/services/cap_grp/cap_grp.c:63:19: note: call to 'realloc' here

63 |         gbuffer = realloc(buf, gbufsize);
   |                   ^~~~~~~~~~~~~~~~~~~~~~

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D42576