Bug fixes

• Fixed a number of small issues found by Coverity.
• When freeing the MMDB struct in MMDB_close() we make sure to set the
  pointers to NULL after freeing the memory they point to. This makes it safe
  to call MMDB_close more than once on the same MMDB_s struct
  pointer. Before this change, calling this function twice on the same pointer
  could cause the code to free memory that belonged to something else in the
  process. Patch by Shuxin Yang. GitHub PR #41.