Fixed seemingly broken if statements in PrintStack

This looked broken, the Xcode analyzer complained about null-pointer dereferences.
Looked to me like the if branches should have been exclusive, i.e., else if branches.

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Tell the analyzer the obvious thing, now we reached zero-complains.

Signed-off-by: Stefan Marr <git@stefan-marr.de>