PE: Fix import parser for non-well-formed import table

[kkent030315]

• Addresses issue PE: import parser is broken for specially crafted binary #428

Changelists:

• Added doc comment for ImportDirectoryEntry::is_null
• Added impl ImportDirectoryEntry::is_possibly_valid to filter out invalid entries where name_rva and import_address_table_rva are both zero. Those never be zero by design if the entry is valid.
• Added integration tests for both well-formed and non-well-formed import table parsing.

The change is straightforward, it does as well as how Windows Loader does. Parser should stop if it is not is_possibly_valid is the same behaviour with Windows Loader and should be more safe than before. This is special exception for non-well-formed case and should not affect well-formed binaries at all.

The same behaviour can be found at systeminformer (formely processhacker) implementation (respectively, peview):
https://github.com/winsiderss/systeminformer/blob/82c625783a035fa7eac355783f527bb53fb1a384/phlib/mapldr.c#L1635

This PR is backward compatible; thus minor changes.

[m4b]

This is great; if you don't mind, since you touched the logs, it'd be nice to see them ported to newer style, but if not, that's fine too!

[m4b]

NB backwards compatible with 0.9