Fix potential crash due to race condition

[kealdishx]

We found some crashes inside kscrash like the following

Exception Type: EXC_BAD_ACCESS ( SIGBUS )
Diagnosis: Attempted to dereference garbage pointer 0x14570808c.Originated at or in a subcall of unknown, cannot find symbol

0 | xxxxxx | ksmc_resumeEnvironment(KSMachineContext.c:231)
-- | -- | --
1 | xxxxxx | CPPExceptionTerminate()(KSCrashMonitor_CPPException.cpp:200)
2 | libc++abi.dylib | std::__terminate(void (*)())()
3 | libc++abi.dylib | ___cxa_throw()
4 | libobjc.A.dylib | _objc_exception_throw()
5 | Foundation | __NSOutOfMemoryErrorHandler()
6 | CoreFoundation | ___CFSafelyReallocate()
7 | Foundation | __NSMutableDataGrowBytes()
8 | Foundation | -[NSConcreteMutableData appendBytes:length:]()
9 | ImageIO | IIOImageWriteSession::putBytes(void const*, unsigned long)()
10 | ImageIO | write_fn(png_struct_def*, unsigned char*, unsigned long)()
11 | ImageIO | _png_write_chunk_data()
12 | ImageIO | __cg_png_write_complete_chunk()
13 | ImageIO | _png_compress_IDAT()
......
22 | libdispatch.dylib | __dispatch_call_block_and_release()
23 | libdispatch.dylib | __dispatch_client_callout()
24 | libdispatch.dylib | __dispatch_queue_override_invoke$VARIANT$armv81()
25 | libdispatch.dylib | __dispatch_root_queue_drain()
26 | libdispatch.dylib | __dispatch_worker_thread3()
27 | libsystem_pthread.dylib | __pthread_wqthread()

It seems the cause is a race condition where g_suspendedThreads was released while still being accessed. My solution is to save suspended threads in local variable instead of global variable, since suspend thread and resume thread are always called in pair.