Fix bug that BE may crash when closing OlapTableSink (#1507)
Fix bug that BE may crash when closing OlapTableSink (#1507)

The `_profile` in OlapTableSink may not be initialized if `prepare()`
method is not called. So when close the OlapTableSink, we should
check if `_profile` is initialized.
imay/palo,imay/palo,imay/palo,imay/palo,imay/palo,imay/palo