Partial fix for zero length parameters.

Suppose one passes n=0 as data to the following model:

    data {
      int<lower=0> n;
    }
    parameters {
      real a0;
      real a[n];
    }
    model {
      a0 ~ normal(0,1);
      a ~ normal(a0,1);
    }

fit.extract() is broken in this case, but fit.summary() and other methods work
fine.