Check if signature from license file is kind of NSData class

Check if `signature` from a license file is kind of `NSData` class to prevent an exception when calling `[signature bytes]`:  

      if (RSA_public_decrypt([signature length], [signature bytes], checkDigest, rsaKey, RSA_PKCS1_PADDING) != 20)
                return nil;

This exception causes `-dictionaryForLicenseData` method to fail, so application's behavior is undefined.    


__Added №1:__  
The same thing with the CoreFoundation-based `AquaticPrime`, afaik.

__Added №2:__  
  
Here is a license file for [«The Tagger»](http://deadbeatsw.com/thetagger/) app which uses this bug so *any user can bypass trial limits*:  
  
    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
    <plist version="1.0">
    <dict>
    	<key>Signature</key>
    	<string>cnc.t3am</string>
    </dict>
    </plist>   
  
*Signature is a string but there isn't any `-bytes` method for `NSString`*.  
(The Tagger uses Cocoa-based implementation of `AquaticPrime`).