seed = NA results in a clang-UBSAN error on CRAN

Hi,

Bit of a niche issue but thought I would post it just to make others aware. Apparently if you set the seed in STAN to NA it results in clang's UBSAN checks (as run by CRAN) throwing an error. A reproducible example can be found by running the following code:

library(rstan)

stancode <- "
data { real y[100]; }
parameters { real mu; real<lower=0> sigma; }
model { y ~ normal(mu,sigma);  }
"

model <- stan_model(model_code = stancode)

fit <- sampling(
    model,
    list(y = rnorm(100, 1.6, 0.2)),
    iter = 200,
    chains = 2,
    refresh = 0,
    seed = NA
)


/opt/R/4.1.3/lib64/R/library/Rcpp/include/Rcpp/internal/caster.h:30:25: runtime error: nan is outside the range of representable values of type 'unsigned int'
    #0 0x7fae25c75e4d  (/tmp/Rtmpb2eTru/file197f05cd19.so+0x2a4e4d)
    #1 0x7fae25c70c57  (/tmp/Rtmpb2eTru/file197f05cd19.so+0x29fc57)
    #2 0x7fae25c46921  (/tmp/Rtmpb2eTru/file197f05cd19.so+0x275921)
    #3 0x7fae2fae733e  (/opt/R/4.1.3/lib64/R/library/Rcpp/libs/Rcpp.so+0x11733e)
    #4 0x7fae36dd6335  (/opt/R/4.1.3/lib64/R/lib/libR.so+0x5b9335)
    #5 0x7fae36f515ba  (/opt/R/4.1.3/lib64/R/lib/libR.so+0x7345ba)
    #6 0x7fae36f3086b  (/opt/R/4.1.3/lib64/R/lib/libR.so+0x71386b)
    #7 0x7fae36f9d429  (/opt/R/4.1.3/lib64/R/lib/libR.so+0x780429)
    #8 0x7fae36f99872  (/opt/R/4.1.3/lib64/R/lib/libR.so+0x77c872)
    #9 0x7fae36f55cd0  (/opt/R/4.1.3/lib64/R/lib/libR.so+0x738cd0)
    #10 0x7fae36f3086b  (/opt/R/4.1.3/lib64/R/lib/libR.so+0x71386b)
    ......

My expectation would have been that an assertion in R throws an error if seed=NA is not supported.

RStan Version:

2.21.3

R Version:

R version 4.1.3 (2022-03-10) - compiled from source using flags specified by the CRAN team

I put together a dockerfile to re-create this environment here

Operating System:

Fedora 34

Start by reproducing the sampling(..., seed = NA) example from the issue, using the linked Fedora CRAN UBSAN Dockerfile if needed. Trace the seed handling around the RStan sampling entry point and the reported Rcpp caster error. Done means seed = NA produces a supported, explicit result without the clang-UBSAN runtime error.