gsl_ran_gaussian_ziggurat doesn't work with gsl_rng_slatec and gsl_rng_uni

[camel-cdr]

I've been developing a ziggurat implementation and used 
https://www.seehuhn.de/pages/ziggurat to benchmark mine against the one used in 
gsl.
I ran[timegauss.c,](https://m.seehuhn.de/data/ziggurat/timegauss.c) but it 
fails with gsl_rng_slatec and gsl_rng_uni.
Consequently, I checked randist/gausszig.c and gsl_ran_gaussian_ziggurat 
doesn't support generators with smaller ranges than 16777216.

On a related note: I'd also be interested in improving the performance of 
gsl_ran_gaussian_ziggurat outside of this bug, but I'm new to this project, and 
I'm not sure what the procedure would be. (In my current benchmarks my 
implementation is 1.3 times faster and uses 2/3 off the memory)