Add bounds checks so that PDO doesn't try to get a column that doesn't exist

Summary: Fixes "Fatal error: Unexpected object type Unknown." errors when calling
fetchColumn() with an invalid column number"

Closes https://github.com/facebook/hhvm/pull/4400

Reviewed By: @JoelMarcey

Differential Revision: D1724093

Signature: t1:1724093:1417921026:3564604feca0969908bc2dedb26098c1397a44b3