avoid to close a closed iterator

[tobecontinued]

Such code will crash.

StorageEngineJungle::KeyItr::close() {
    if (itr) {
        itr->close();
        DELETE(itr);
    }
    if (snap) {
        DB::close(snap);
        snap = nullptr;
    }
}

27 0x0000563511a97527 in SimpleLoggerMgr::handleSegFault (sig=11) at

28

29 0x0000563511f9202b in jungle::Iterator::close (this=this@entry=0xd856) at

30 0x0000563511f92129 in jungle::Iterator::~Iterator (this=0xd856, __in_chrg=)

[greensky00]

@tobecontinued Calling Iterator::close() multiple times should be fine because 1) AVL-tree is already empty, and 2) itrs is already cleared. It will simply do nothing. This has been tested (calling close and then once again by the destructor) in many functional test cases. Can you investigate with GDB what caused the crash?

[greensky00]

Closing this PR as double-closing of iterator has no problem.