powm with zero modulus raises sigfpe

[TyOverby]

$ utop
utop # Zarith.Z.(powm (of_string "-27") (of_string "1") (of_string "0"));;
Floating point exception (core dumped)
$ 

[antoinemine]

Thanks for the report! PR #60 has been updated to fix this (as well as some memory leaks when exceptions are raised).

[antoinemine]

PR merged !