Check undefined skip check slave lag schema change

[cswingler]

The P parameter can be undefined in DSNs, resulting in the skip-check-slave-lag feature in pt-table-checksum puking trying to compare undefs.

This handles that check a little more safely.

This is exactly like #230 , but for pt-online-schema-change.

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:white_check_mark: svetasmirnova
:x: Chris Swingler

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[hhgyu]

the issue still exists.

[svetasmirnova]

We may need to check if other tools require similar fix.