Ensure that we release CoreGraphics objects.
Ensure that we don't attempt to use a nil navigation item in an array.

This could happen if there isn't a right placeholder controller and the right view controller was also nil prior to the pop operation.
More explicit conditionals so LLVM can analyze nil value flow.
Merge pull request #14 from jcoleman/fix-analyzer-warnings-and-bugs

Fix LLVM analyzer warnings and bugs