This fixes a regression in 73132a6 (#200) where nil was returned
from a retain call after the object begins deallocating. Normal
retains of a deallocating object are still expected to return an
instance pointer inside its dealloc method and code compiled with ARC
will generate calls to objc_storeStrong() when a block captures the
self pointer inside the dealloc method.
The previous checks for a deallocating object with negative reference count did not work because:
1. the sign bit has to be recreated as was happening in objc_delete_weak_refs()
2. there was no distinction between a saturated count and a negative reference count
refcount_max now indicates when the refcount has saturated and should no longer be mutated.
An underflow to -1 still maps to refcount_mask, allowing us to detect when an object is supposed to be deallocated.
Neither objc_release_fast_no_destroy_np() nor objc_retain_fast_np() mutate the refcount when it is one of those values, so the comment in objc_delete_weak_refs() was adjusted.
Graham--M