Less surprising behaviour.

Make redisFree() and freeReplyObject() less surprising by behaving just
like free(). That is, don't crash when passing in NULL.
Add tests for redisFree() and freeReplyObject()

Test that redisFree() and freeReplyObject() do not crash when NULL is
passed in.
Merge pull request #246 from dmelani/master

Less surprising behaviour in redisFree() and freeReplyObject()