Fixing two project errors stoping correct compilation of the iOS project in XCode 7

[aardvarkk]

One error was due to comparing a pointer with a boolean -- switched to comparing to nullptr instead.
The second error was that, for some reason, the project was referencing SHA1.cpp (which didn't exist), so I added in DR_SHA1.cpp instead (which did exist and fixed the linking errors).

[Luke1410]

The first change is a duplicate of #29 (which was already incorporated in SLikeNet 0.1.0). The second change I just incorporated in SLikeNet (SLikeSoft#34 - internal case number SLNET-188) and is available in the master branch. It is expected to get shipped with SLikeNet 0.2.0.

[Luke1410]

Due to lack of other means to contact you, posting this here as a comment:

I understand that you didn't put your pull request under a special license and hence it's covered by the original RakNet license. Please let me know if I got this wrong, and you're rather considering your contributions under special license terms.

As a way to thank you for your contribution, we'd like to offer you putting you in the acknowledgments section of the accompanying readme for the next version:

• Ian Clarkson: https://github.com/aardvarkk (pull request: RAKNET_84)

Please let me know if that's fine with you (preferably by mail to info [at] slikesoft [dot] com - or alternatively leave a comment here). Unless you explicitly say so, we won't put that entry in the readme file (to respect your privacy).

[aardvarkk]

Original RakNet license is fine. Sure -- adding me to the README would be great! Thanks for commenting on the issue with your updates, I appreciate it.

[Luke1410]

Thanks for providing the contribution. :-) --- I just added the entry and it will be in the next SLikeNet version.