Avoid selecting RegionAllocator with invalid arguments in allocate()

Summary:
We don't actually use the region allocator if the arguments are invalid, so don't get it unless we actually use it.

The compiler *hopefully* already optimizes this away, but just ensure.

Reviewed By: AlnisM

Differential Revision: D88484601

fbshipit-source-id: 069d8659265e9212a25a9eacf8ba5cad0365a737

Author: Rob Lyerly

cachelib/navy/block_cache/Allocator.cpp

@@ -51,12 +51,12 @@ Allocator::Allocator(RegionManager& regionManager,
 std::tuple<RegionDescriptor, uint32_t, RelAddress> Allocator::allocate(
     uint32_t size, uint16_t priority, bool canWait, uint64_t keyHash) {
   XDCHECK_LT(priority, allocators_.size());
-  RegionAllocator* ra =
-      &allocators_[priority][keyHash % allocators_[priority].size()];
   if (size == 0 || size > regionManager_.regionSize()) {
     return std::make_tuple(RegionDescriptor{OpenStatus::Error}, size,
                            RelAddress());
   }
+  RegionAllocator* ra =
+      &allocators_[priority][keyHash % allocators_[priority].size()];
   return allocateWith(*ra, size, canWait);
 } // namespace cachelib