Prevent divide by zero errors in ObjSize controller.

Summary:
AdRanker ASAN canary flagged a possible UBSan violation.

## Error
Failed Run: https://fburl.com/servicelab/apytosry
```
    #0 0x562e3adb59bc in facebook::cachelib::objcache2::ObjectCacheSizeController<facebook::cachelib::CacheAllocator<facebook::cachelib::LruCacheTrait> >::work() buck-out/v2/gen/fbcode/47d914adeee3d982/cachelib/experimental/objcache2/__object-cache__/headers/cachelib/experimental/objcache2/ObjectCacheSizeController-inl.h
    #1 0x562de7610f78 in facebook::cachelib::PeriodicWorker::loop() fbcode/cachelib/common/PeriodicWorker.cpp:55
    #2 0x7f7632c524e4 in execute_native_thread_routine /home/engshare/third-party2/libgcc/11.x/src/gcc-11.x/x86_64-facebook-linux/libstdc++-v3/src/c++11/../../../.././libstdc++-v3/src/c++11/thread.cc:82:18
    #3 0x7f7632f6ec0e in start_thread /home/engshare/third-party2/glibc/2.34/src/glibc-2.34/nptl/pthread_create.c:434:8
    #4 0x7f76330011db in clone3 /home/engshare/third-party2/glibc/2.34/src/glibc-2.34/misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

UndefinedBehaviorSanitizer: integer-divide-by-zero buck-out/v2/gen/fbcode/47d914adeee3d982/cachelib/experimental/objcache2/__object-cache__/headers/cachelib/experimental/objcache2/ObjectCacheSizeController-inl.h:33:40 in
```

Reviewed By: jiayuebao

Differential Revision: D39024188

fbshipit-source-id: 64ad644c360565e638fa3ca74616a315038382ab

Author: Swaroop Manjunath

cachelib/experimental/objcache2/ObjectCacheSizeController-inl.h

@@ -21,8 +21,11 @@ namespace cachelib {
 namespace objcache2 {
 template <typename AllocatorT>
 void ObjectCacheSizeController<AllocatorT>::work() {
-  auto totalObjSize = objCache_.getTotalObjectSize();
   auto currentNumEntries = objCache_.getNumEntries();
+  if (currentNumEntries == 0) {
+    return;
+  }
+  auto totalObjSize = objCache_.getTotalObjectSize();
   // Do the calculation only when total object size or total object number
   // achieves the threshold. This is to avoid unreliable calculation of average
   // object size when the cache is new and only has a few objects.