Fixed query checker for SQLDatabaseChain (#4780)
# Fixed query checker for SQLDatabaseChain
When `SQLDatabaseChain`'s llm attribute was deprecated, the query
checker stopped working if `SQLDatabaseChain` is initialised via
`from_llm` method. With this fix, `SQLDatabaseChain`'s query checker
would use the same `llm` as used in the `llm_chain`
## Who can review?
@hwchase17 - project lead
Co-authored-by: imeckr <chandanroutray2012@gmail.com>
