Misc bug fixes in parse_args
Don't use assert to check hamming_mutate_dna
Fix another coverity bug

Do an extra dereference check in the exit section of load_tries_single,
as on mutated == NULL, we goto exit;. This means we need to deal with
the case of mutated being a null pointer, and not dereference it.
Remove zlib as submodule

Accidentally added it in last commit
[README] Section on updating zlib; misc formatting