Revert "[Issue #1592] Replacing NSParameterAssert with early nil return if session was unable to create a task"

The underlying cause was that NSURLSession was nil.

This reverts commit 14e1b7881ad5fa9f9cbb37297b11734f72b673bb.
Merge branch 'master' of github.com:AFNetworking/AFNetworking
Removing intermediary variable