vm.c: refresh ci after mrb_const_set in OP_SETCONST

mrb_const_set may invoke const_added via mrb_funcall_argv, which
re-enters the VM and can reallocate cibase. This invalidates the
local ci pointer (and thus the regs macro that expands to ci->stack),
causing a use-after-free on the next opcode dispatch.

OP_SETMCNST and the OP_GET* / OP_*IDX opcodes already follow this
pattern; align OP_SETCONST with them.

Reported by OSS-Fuzz (clusterfuzz testcase 5886006653157376).

Co-authored-by: Claude <noreply@anthropic.com>

src/vm.c

@@ -2477,6 +2477,7 @@ RETRY_TRY_BLOCK:
       struct RClass *c = MRB_PROC_TARGET_CLASS(ci->proc);
       if (!c) c = mrb->object_class;
       mrb_const_set(mrb, mrb_obj_value(c), irep->syms[b], regs[a]);
+      ci = mrb->c->ci;
       NEXT;
     }