msMSSQL2008LayerGetExtent doesn't consider NATIVE_FILTER or translated filter

The postgis and the oracle drivers limit the extent query within the specified filter (either by using NATIVE_FILTER or translated filter). We should also establish the same option for the mssql driver.

Also msMSSQL2008LayerGetExtent  causes a crash if the queries return null values. ie. when SQLGetData return -1 in the retLen parameter. 