dynamorio could crash when clone syscall's newsp == 0

Created by: hac425xxx

when newsp == 0, we should save the mc->sp to record->app_thread_xsp

Bug fix

core/unix/signal.c

@@ -632,6 +632,13 @@ create_clone_record(dcontext_t *dcontext, reg_t *app_thread_xsp)
        record = (clone_record_t *)(dstack - sizeof(clone_record_t));
        ASSERT(ALIGNED(record, get_ABI_stack_alignment()));
        record->app_thread_xsp = *app_thread_xsp;

        /* fix bug when  clone syscall's argument: newsp == 0  (for example: vfork in android libc.so)
         */
        if(record->app_thread_xsp == 0)
        {
            record->app_thread_xsp = get_mcontext(dcontext)->sp;
        }
        /* asynch_target is set in d_r_dispatch() prior to calling pre_system_call(). */
        record->continuation_pc = dcontext->asynch_target;
        record->clone_flags = dcontext->sys_param0;