nfeltpowmodpr segfault on degree-1 ideals

[Bill Allombert <allomber@math.u-bordeaux.fr>]

Dear PARI dev

It seems that nfeltpowmodpr SEGV on degree 1 ideal:
? K=nfinit(x^2+1);pr=nfmodprinit(K,idealprimedec(K,5)[1]);
? nfeltpowmodpr(K,x,2,pr);
  ***   bug in GP (Segmentation Fault), please report

For some reason to_ff_init return NULL for the polynomial,
but FpXQ_pow will not handle it.

Cheers,
Bill