Add null checks on driver PDO statement

Summary: php5 makes all methods do nothing if there is no backing driver PDO statement
Closes https://github.com/facebook/hhvm/pull/4064

Reviewed By: @paulbiss

Differential Revision: D1640296

Signature: t1:1640296:1414598532:2f284f59c76fa94b926251aabe2d27cd0a7de470

Pulled By: @ptarjan