Description of problem:
ifup-eth doesn't properly handle the case where a mac-address can't be found, this occurs if you have replaced the network card.

relevant section from ifup-eth0:
# remap, if the device is bound with a MAC address and not the right device num
# bail out, if the MAC does not fit
if [ -n "${HWADDR}" ]; then
    FOUNDMACADDR=`get_hwaddr ${REALDEVICE}`
    if [ "${FOUNDMACADDR}" != "${HWADDR}" ]; then
        curdev=`get_device_by_hwaddr ${HWADDR}` # Returns empty if $HWADDR can't be found
        if [ -n "$curdev" ]; then # Returns false since $HWADDR is empty,  
	  rename_device "${REALDEVICE}" "${HWADDR}" "${curdev}" || {
	    echo $"Device ${DEVICE} has different MAC address than expected, ignoring." # This else clause should be moved up to the if statemetn
	    exit 1
	  }
	fi
    fi
fi

How reproducible:
1. ifdown $if
2. edit config, change HWADDR to a nonexisting mac address on the system.
3. ifup $if # Should scream about mac address not found


Actual results:
Interface goes up


Expected results:
Should not go up.

Additional info:
Tested on 5.2 and 5.3