A Grand Prairie police officer and another driver are very fortunate to be alive after a  violent rear-end crash on I-30 near Belt Line late last night.  The rear end of the patrol car was obliterated when a pick-up smashed into it.  But both the officer and the pick-up driver were un-hurt, despite the tremendous damage.  Investigators say the officer pulled onto the shoulder a little past 11 p.m. Monday so he could remove a mattress that had fallen onto the freeway.  The pick-up driver may have been trying to avoid the mattress when she swerved onto the shoulder and rear-ended the patrol car.  Police don’t expect any charges against her.