PE: handle empty import address table

[williballenthin]

This PE file has a valid import lookup table but the pointer to the import address table is 0x0:

goblin currently supports handling empty import lookup tables but not empty import address tables:

goblin/src/pe/import.rs

Lines 189 to 191 in 9f7fb6b

	pub import_lookup_table: Option<ImportLookupTable<'a>>,
	/// Computed
	pub import_address_table: ImportAddressTable,

Could we extend goblin to handle the above case (parse synthentic entries from the import lookup table when the import address table is 0x0)?

[m4b]

If you understand the issue and might be willing to fix, a PR would be appreciated :)