Fix within() bugs that caused nodes skipping:

• When startNode() returns a data pointer, the code skipped it because of "if (node < node_count)".
• When traversing the prefix, if a data record is encountered before exhausting all prefix bits (querying /24 but record is at /23), the code skipped the data pointer.

Add IP address validation to within() and lookup: when IPv6 address was used on IPv4-only database, the results could be garbage, so such address is rejected now.

Full Changelog: v0.3.0...v0.4.0