Fix an edge case in find_primroot()

conditions :  prime == 257 and factor[] = [2] and candidate inital value == 256
on first loop check_coprime() will return NOT_COPRIME
line 117 ++candidate will make candidate==257
on second loop check_coprime() will return COPRIME
and then in isomorphism()  assert(additive_elt(257) < mult_group->prime(257)); will fail
Solution : reset candidate to minimal value (1)