[FSA] Fix use-after-free in cleanup

If we cleanup the main thread (with it's per-thread FSA free list) after
cleaning up the FSA itself, that corresponds to a use-after-free, and we
should not.

src/moar.c

@@ -588,8 +588,6 @@ void MVM_vm_destroy_instance(MVMInstance *instance) {
     uv_mutex_destroy(&instance->nfg->update_mutex);
     MVM_nfg_destroy(instance->main_thread);
 
-    /* Clean up fixed size allocator */
-    MVM_fixed_size_destroy(instance->fsa);
 
     /* Clean up integer constant and string cache. */
     uv_mutex_destroy(&instance->mutex_int_const_cache);
@@ -603,6 +601,9 @@ void MVM_vm_destroy_instance(MVMInstance *instance) {
     MVM_tc_destroy(instance->main_thread);
     uv_mutex_destroy(&instance->mutex_threads);
 
+    /* Clean up fixed size allocator */
+    MVM_fixed_size_destroy(instance->fsa);
+
     /* Clear up VM instance memory. */
     MVM_free(instance);
 }