Fix a problem with ord_lookup.

If the ordinal can not be looked up we must return "ordN" and not NULL.

This was causing things with imports by ordinal that do not map to have
incorrect hashes. For example, this file:

d4ffa4559a1e22167933772d82cf714cd4bb7a0e79511c2424e18bdb619d63a4

The correct imphash for this (according to pefile) is:

74638b0911b0c9268aae1edee5a60a9f
Merge pull request #199 from wxsBSD/ord_lookup_fix

Fix a problem with ord_lookup.