If the Human race were to colonise Mars, it is equally likely that an impact on Mars could destroy civilisation there as for the folks "back home". No planet is safer than any other. The colonists who go to Mars face many dangers as well as the threat of major impact. Their risks are far higher than those who stay and they do not diminish the survival chances of Earthlings merely by going. Mankind has the resources to BOTH go to Mars AND learn about NEO's.

In fact with two planets the odds of a disater are doubled (from 1 in 1 billion to 2 in 1 billion), but the odds of survival are increased enormously (from 1-{1 in 1 billion} to 1-{1 in 1 billion billion}) .

Simple statistics.

In the process of developing our spacefaring capacity enough to go to Mars, we will also enable a major NEO (and NMO) detection and mitigation strategy.

Part of the defence against NEO's is detection - which we can do now. Part is dispersion of the target - which we are beginning to think about seriously. Part is active countermeasures which we are also thinking about but may never be 100% assured due to the implacable celestial physics of long-period comets which are hard to detect, unpredictable, and almost unstoppable.