Fix caches manager operation thread safe issue

SDWebImage/Private/SDImageCachesManagerOperation.m

@@ -9,10 +9,22 @@
 #import "SDImageCachesManagerOperation.h"
 
 @implementation SDImageCachesManagerOperation
+{
+    dispatch_semaphore_t _pendingCountLock;
+}
 
 @synthesize executing = _executing;
 @synthesize finished = _finished;
 @synthesize cancelled = _cancelled;
+@synthesize pendingCount = _pendingCount;
+
+- (instancetype)init {
+    if (self = [super init]) {
+        _pendingCountLock = dispatch_semaphore_create(1);
+        _pendingCount = 0;
+    }
+    return self;
+}
 
 - (void)beginWithTotalCount:(NSUInteger)totalCount {
     self.executing = YES;
@@ -20,8 +32,17 @@
     _pendingCount = totalCount;
 }
 
+- (NSUInteger)pendingCount {
+    SD_LOCK(_pendingCountLock);
+    NSUInteger pendingCount = _pendingCount;
+    SD_UNLOCK(_pendingCountLock);
+    return pendingCount;
+}
+
 - (void)completeOne {
+    SD_LOCK(_pendingCountLock);
     _pendingCount = _pendingCount > 0 ? _pendingCount - 1 : 0;
+    SD_UNLOCK(_pendingCountLock);
 }
 
 - (void)cancel {
@@ -36,7 +57,9 @@
 }
 
 - (void)reset {
+    SD_LOCK(_pendingCountLock);
     _pendingCount = 0;
+    SD_UNLOCK(_pendingCountLock);
 }
 
 - (void)setFinished:(BOOL)finished {