First of all, the standards in 1970 were quite a bit higher for planetary protection of Mars than they are today. The Viking Landers were sterilized in a giant oven, a step which is not done today. We can build a lander to the same standards as an orbiter back in the day, which basically involve constructing in a clean room. In the 1970s things were sterilized completely that landed.
It is entirely possible that Viking 1 is still orbiting Mars, or it may have impacted. We just don't know. The way to figure this out would be to somehow search the orbit of Mars for a small object, which is theoretically possible using the spacecraft orbiting Mars now. They would be able to find a tiny asteroid orbiting, or a dead spacecraft. This hasn't been done, it just isn't considered enough of a priority to make it happen.