bugfix: test/test_hashtable segment fault

In zc_hashtable.c, zc_hashtable_remove() and zc_hashtable_del() didn't check the
argument pass to it.  So, if pass NULL to them, they will dereference it, and if
we are lucky, it will segment fault.

This commit fix that problem.
Merge pull request #39 from mofaph/bugfix-38

尝试修复 #38 出现的段错误