Fix uninitialized-variable error

The version of Clang in Xcode 4.6 now complains about usage of an
uninitialized variable 'hasBytesAvailable' in -[GCDAsyncSocket doReadData].
This breaks compilation. I've added a few lines to initialize the variable.
Merge pull request #57 from couchbaselabs/master

Fix uninitialized-variable error with Xcode 4.6+