fix segfault in seqtk sample with large N

If the <number> given to seqtk sample is too large,
    calloc(num, sizeof(kseq_t);
will fail, leading to a segfault in cpy_kseq, where buf (which is ==
NULL) is dereferenced.

This fixes this.
use inttypes to silence warning w/ printf uint64_t
Merge pull request #24 from kdmurray91/fixsigsegv

fix segfault in seqtk sample with large N