Fix CPUCaching allocator guard bug (#46922)

Summary:
Pull Request resolved: #46922

Earlier bug wrongly captures the previous value to be saved.

Test Plan: cpu_caching_allocator_test

Reviewed By: dreiss

Differential Revision: D24566514

fbshipit-source-id: 734a4c1f810bbec16fe007f31fffa360898955ac

c10/mobile/CPUCachingAllocator.cpp

@@ -95,8 +95,8 @@ CPUCachingAllocator* GetThreadLocalCachingAllocator() {
 
 WithCPUCachingAllocatorGuard::WithCPUCachingAllocatorGuard(
     CPUCachingAllocator* allocator) {
-  caching_allocator_ptr = allocator;
   prev_caching_allocator_ptr_ = GetThreadLocalCachingAllocator();
+  caching_allocator_ptr = allocator;
 }
 
 WithCPUCachingAllocatorGuard::~WithCPUCachingAllocatorGuard() {