scfmDriver fails if every cell is flammable

[ianmseddy]

Per Chris Mellon, this is an issue mainly for simulated landscapes, but the following line in driverFunctions.R will error if there are no non-flammable pixels.

    res <- optimise(escapeProbDelta,
                    interval = c(hatP0(hatPE, neighbours),
                                 hatP0(hatPE, floor(sum(w * 0:8)))),
                    tol = 1e-4,
                    w = w,
                    hatPE = hatPE)