Fix 'caml_unregister_frametable(table)' when 'table' is the first element

[recoules]

Closes #11633.

[Ekdohibs]

This looks correct to me!

[gasche]

@Ekdohibs thanks! Do you know if this bug also requires a fix in 5.0?

(cc @Octachron: I'm happy to approve on @Ekdohibs' behalf, but can we include new fixes in 4.14 now that a RC is out?)

[Octachron]

We are indeed lacking a development branch for 4.14 right now. It would be better to merge the PR after the 4.14.1 release in less than two weeks. Then the fix will be part of the 4.14.2 release.

[gasche]

(If there is something to fix in trunk, possibly by just rebasing the proposed patch, we can merge it right now; ideally someone would do a separate PR for this.)

[Ekdohibs]

I'm not sure if there are any changes to do in trunk, since the logic for freeing frame descriptors changed quite a lot:

ocaml/runtime/frame_descriptors.c

Lines 195 to 208 in b06f2d1

	/* it's now safe to free the old table(s) */
	caml_plat_lock(&descr_mutex);
	if (ft->prev != NULL) {
	struct frametable_version *p = ft->prev;
	while (p != NULL) {
	struct frametable_version *next = p->prev;
	caml_stat_free(p->table.descriptors);
	caml_stat_free(p);
	p = next;
	}
	ft->prev = NULL;
	atomic_store_rel(&ft->free_prev_after_cycle, No_need_to_free);
	}
	caml_plat_unlock(&descr_mutex);

[nojb]

This needs a Changes entry I think.