Fix: property access is not allowed yet

Fixes the warning emitted by mysqli when sqlstate is not set (yet?).
http://git.php.net/?p=php-src.git;a=blob;f=ext/mysqli/mysqli_prop.c;h=2d36336372b75922bd8fbf40c5c9054a5230c8a0;hb=HEAD#l36

Warning masks actual connection error.

Related:
http://www.doctrine-project.org/jira/browse/DBAL-911