Fixed a potential access violation

[schen0x]

Add an option to not parse the content of ExportedDirectory when target dll is an image, because:

• The ExportedDirectory (VA) is an address in .rdata;
• The address is de-referenced in the getExportedAddressTable and 3 other assembly functions;
• parseDLL() is called on the raw_beacon_dll, which is an image;
• However, this address may not exist on an image (because this may be a big address, but image is not loaded) thus may trigger an access violation. Specifically, when "ExportedDirectoryRVA" > "imageSize".
  (Not sure if there is a better solution)
• The modification should not effect other code, i.e when parsing an in-memory dll; but will parse only the header when the dll is an image (probably another solution is to add auto-detection of image-only header (e.g PE\0\0) but it could be an extra point of failure?) because the data will be garbage anyway and may trigger an alarm.