There are an estimated 150,000 car fires in the US every year. I don't think either of us has the data available to make an apples to apples comparison but I very much doubt that battery powered cars will prove to be meaningfully more hazardous that gasoline powered ones.
Gasoline does not have to be in a combustion chamber to ignite. A hot manifold with a leaking fuel line is more than enough to set a car on fire.
My point: Fuel cell batteries where; One could refill a tank of reactant and having the reactor part in a distinct area, would not need having the chemical reaction so close to the stored energy (thin layers in lithium batteries). About burning cars: If ppls with electric cars were skipping maintenance and having so much mishandling than those burning IC engine cars endured. I think consequences would be way more dramatic. Be sure as soon as Electric Cars becomes affordable to the mass, there will be enough fools to cross the boundaries of safety.