try fix segfault on client (async UAF?)

[eagleoflqj]

I guess the problem is that IC may be destroyed before the code deferred to main thread is executed. So I make the swift function async instead of the C++ function.
While I have no guarantee that it can avoid the crash, I do believe we won't see the same stacktrace again.

Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       KERN_INVALID_ADDRESS at 0x00005741e5894870
Exception Codes:       0x0000000000000001, 0x00005741e5894870

Termination Reason:    Namespace SIGNAL, Code 11 Segmentation fault: 11
Terminating Process:   exc handler [742]

VM Region Info: 0x5741e5894870 is not in any region.  Bytes after previous region: 95927650568305  Bytes before following region: 3914568480656
      REGION TYPE                    START - END         [ VSIZE] PRT/MAX SHRMOD  REGION DETAIL
      VM_ALLOCATE (reserved)      2f8000000-300000000    [128.0M] rw-/rw- SM=NUL  reserved VM address space (unallocated)
--->  GAP OF 0x5ace54000000 BYTES
      JS JIT generated code    5ad154000000-5ad154001000 [    4K] ---/rwx SM=NUL  

Thread 0 Crashed::  Dispatch queue: com.apple.main-thread
0   libswiftCore.dylib            	    0x7ff815e777f9 swift_unknownObjectRetain_n + 41
1   Fcitx5                        	       0x105d3d949 commitAndSetPreedit(_:_:_:_:_:) + 57
2   Fcitx5                        	       0x105db954c invocation function for block in fcitx::MacosInputContext::commitAndSetPreeditAsync() + 140
3   libdispatch.dylib             	    0x7ff8052e4ac6 _dispatch_call_block_and_release + 12
4   libdispatch.dylib             	    0x7ff8052e5dbc _dispatch_client_callout + 8
5   libdispatch.dylib             	    0x7ff8052f21df _dispatch_main_queue_drain + 984
6   libdispatch.dylib             	    0x7ff8052f1df9 _dispatch_main_queue_callback_4CF + 31
7   CoreFoundation                	    0x7ff8055a3866 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9
8   CoreFoundation                	    0x7ff805562a95 __CFRunLoopRun + 2459
9   CoreFoundation                	    0x7ff805561b32 CFRunLoopRunSpecific + 557
10  HIToolbox                     	    0x7ff80ff73829 RunCurrentEventLoopInMode + 292
11  HIToolbox                     	    0x7ff80ff73636 ReceiveNextEventCommon + 665
12  HIToolbox                     	    0x7ff80ff73381 _BlockUntilNextEventMatchingListInModeWithFilter + 66
13  AppKit                        	    0x7ff808bc9be5 _DPSNextEvent + 880
14  AppKit                        	    0x7ff8094d9fe9 -[NSApplication(NSEventRouting) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1273
15  AppKit                        	    0x7ff808bbb005 -[NSApplication run] + 603
16  AppKit                        	    0x7ff808b8eff1 NSApplicationMain + 816
17  Fcitx5                        	       0x105c9ddfc main + 28
18  dyld                          	    0x7ff8050fb366 start + 1942

[ksqsf]

I agree with your analysis. Let's give it a try.