Fix a crash caused by private class _PFResultASCIIString in iOS 14

[xushuangqing]

Hello,

When using FBRetainCycleDetector on iOS 14, I got frequent crash. The stack is:

MACH_Exception EXC_BAD_ACCESS KERN_INVALID_ADDRESS fault_address:0x000000014521c678
0libobjc.A.dylib
_objc_loadWeakRetained (in libobjc.A.dylib)
1 MyApp                -[FBObjectiveCGraphElement object] (in NewsInHouse) FBObjectiveCGraphElement.h:41
2 MyApp                -[FBObjectiveCGraphElement isEqual:] (in NewsInHouse) FBObjectiveCGraphElement.mm:91
3 MyApp                -[FBNodeEnumerator isEqual:] (in NewsInHouse) FBNodeEnumerator.mm:51
4 CoreFoundation -[__NSSetM containsObject:] (in CoreFoundation)
5 MyApp                -[FBRetainCycleDetector _findRetainCyclesInObject:stackDepth:] (in NewsInHouse) FBRetainCycleDetector.mm:117
6 MyApp                -[FBRetainCycleDetector findRetainCyclesWithMaxCycleLength:] (in NewsInHouse) FBRetainCycleDetector.mm:65

It typically happened when the candidate is a PHAsset object. When FBObjectiveCGraphElement->_object becomes a _PFResultASCIIString, crash happened.

in iOS 14, PHAsset has instance variables whose classes are _PFResultASCIIString. Below is one PHAsset's retained objects:

Printing description of retainedObjects:
<__NSArrayM 0x282aac270>(
-> _cloudAssetGUID -> _PFResultASCIIString ,
-> _creationDate -> __NSTaggedDate ,
-> _modificationDate -> __NSTaggedDate ,
-> _uniformTypeIdentifier -> _PFResultASCIIString ,
-> _directory -> _PFResultASCIIString ,
-> _filename -> _PFResultASCIIString ,
-> _imageRequestHints -> _PFResultData ,
-> _faceAdjustmentVersion -> __NSTaggedDate ,
-> _uuid -> _PFResultASCIIString ,
-> _objectID -> (null) ,
-> _photoLibrary -> PHPhotoLibrary 
)

_PFResultASCIIString may be a new private class from iOS 14. It is a subclass of NSString, but it has some strange behaviors. As malloc_zone_from_ptr(pfResultASCIIString) returns null. It seems _PFResultASCIIString is not a malloced object.

I suppose, if an object is not malloced/realloced, checking by FBRetainCycleDetector is meaningless. So I added a condition to filter objects that are not malloced.