Issue: the function get_num_upars to access the number of unconstrained parameters from a stan objecti does not work with previously saved stan object

- Test Model and Data

```
modelcode <- "
data{
int<lower=0> N; // number of data points
real Y[N]; //
}
parameters{
real mu;
real<lower=0> eta;
}
transformed parameters{
real<lower=0> sigma;
sigma = 1 / eta;
}
model{
mu ~ normal(0,10);
sigma ~ cauchy(0,1);
}
generated quantities{
real Yhat[N];
for(n in 1:N){
Yhat[n] = Y[n] + 1;
}
}
"
model <- stan_model(model_code = modelcode)
dataList <- list(
N=50,
Y=rnorm(50,0,1)
)
fit <- sampling(object = model, data = dataList)
get_num_upars(fit)
# number of parameters are returned as expected, in this case:2
```

- Problem

The stan object can be saved in multiple ways:

```
# method 1: save the stan object as .rdata
save(list="fit",file = "savefile.rdata")
load("savefile.rdata") # load stan object "fit" into global environment
# method 2: save as rds
saveRDS(fit,"fit.rds")
fit1 <- readRDS("fit.rds") # create a new stan object "fit1" from saved RDS file
# get_num_upars does not work with either
get_num_upars(fit1)
# an error is returned: the model object is not created or not valid
```

A list of actions are mentioned at Access contents of stanfit object. These actions can be normally performed with the saved object fit1 without any problems.