Fix stringop-overflow warning in Scratch.h (facebookincubator#9526)

Summary:
Fixes: facebookincubator#9525

Compiler generates stringop-overflow warning because it fails to infer that
`if (newCapacity > capacity_)` would be false when `newCapacity` is zero.

Add hint to prevent the compiler from generating the stringop-overflow
warning when 'newCapacity' is 0.

Pull Request resolved: facebookincubator#9526

Reviewed By: xiaoxmeng

Differential Revision: D56308183

Pulled By: Yuhta

fbshipit-source-id: a41f0060c11ad906c1e91ff3469361e837d5d00d

velox/common/base/Scratch.h

@@ -75,6 +75,9 @@ class Scratch {
     for (auto i = newCapacity; i < fill_; ++i) {
       std::destroy_at(&items_[i]);
     }
+    // Add hint to prevent the compiler from generating the
+    // stringop-overflow warning when 'newCapacity' is 0.
+    folly::assume(capacity_ >= 0);
     if (newCapacity > capacity_) {
       Item* newItems =
           reinterpret_cast<Item*>(::malloc(sizeof(Item) * newCapacity));