fix rabin-miller witness limit typo (s/2027/2047/), and don't return …

…1 as a prime, which is more conventional and makes it easier to compare output with primes(3) utility

std::string and realloc don't mix; fixes Issue wahern#6, Crash in PHF…

…::init<std::string>

removed unused variable, k, in phf_rabinmiller