Initialized variable hasBytesAvailable to fix a warning for OS X target.

Trying to silent a warning raised when using for an OS X target. The thing that is happening is the macro ```#if TARGET_OS_IPHONE``` (line 4252) is blocking the initialization of `hasBytesAvailable` lines 4258 or 4260, then causing that warning.

I know that the condition ```[self usingCFStreamForTLS]``` is always false, so it shouldn't cause any random bug, but it was just to keep it warning-free.

Thanks for this truly awesome library :)

Micha