Issue #136: Wrong data value returned in Row in case of non-existent index

When using a non-existing array offset to get from the Row, array_search will return false. This will be typecasted into a int and offset 0 will be returned instead of the requested.

This fixes so if array_search() returns false, offsetGet() also returns false.

The existing testcases did not detect this bug since they used isset() for the non-existing keys, resulting in the usage of offsetExists(), not offsetGet().