Fixed two issues with our error handling of strtol

* If you used a non-integer string as an array index when doing a lookup
* with `MMDB_get_value`, `MMDB_vget_value`, or `MMDB_aget_value`, the first element
  of the array would be returned rather than an error. A
  `MMDB_LOOKUP_PATH_DOES_NOT_MATCH_DATA_ERROR` error will now be returned.
* If a number larger than `LONG_MAX` was used in the same functions,
  `LONG_MAX` would have been used in the lookup. Now a
  `MMDB_INVALID_LOOKUP_PATH_ERROR` error will be returned.
* I added tests for both of these as well as a missing test for a negative
  array index.
Remove a pointless memset() call

We were already memset()ing the entry_data structure in MMDB_aget_value if
lookup_path_in_array doesn't return a success status.
Merge pull request #67 from maxmind/greg/handle-strtol-errors

Fixed two issues with our error handling of strtol