Google "Dog eats Owner"
CNN
Pravda
Mail Online
It doesn't appear to be common, but then the circumstances in which this occurs aren't real common either as it requires someone dying in a confined area with a dog and not being discovered before the dog runs out of better pickings.
Though google also seems full of some crazy story about a dog saving a diabetic man by eating his toes (though links seems to disagree on where this happened and the number of toes)