libasm: Fix use-after-free issue with circular single linked list cleanup

libasm: Fix use-after-free issue with circular single linked list cleanup

Pointed out by gcc 12 with -Wuse-after-free=3

In function ‘free_section’
asm_end.c:552:17: error: pointer ‘data’ used after ‘free’ [-Werror=use-after-free]
  552 |     while (oldp != scnp->content);
      |            ~~~~~^~~~~~~~~~~~~~~~
asm_end.c:550:9: note: call to ‘free’ here
  550 |         free (oldp);
      |         ^~~~~~~~~~~

Fix by freeing scnp->content last.

Signed-off-by: Mark Wielaard <mark@klomp.org>