Silence minor compiler warnings.

Ensure that ClassifyUtilityCommandAsReadOnly() has defined behavior
even if TransactionStmt.kind has a value that's not one of the
declared values for its enum.

Suppress warnings from compilers that don't know that elog(ERROR)
doesn't return, in ClassifyUtilityCommandAsReadOnly() and
jsonb_set_lax().

Per Coverity and buildfarm.