folly: fbvector: ubsan: don't call memcpy() with nullptr args if size…

… == 0

Reviewed By: meyering, luciang

Differential Revision: D3310683

fbshipit-source-id: c86471d54062b2f3455f15fb73340fac486d9e44

folly/FBVector.h

@@ -522,15 +522,19 @@ class fbvector : private boost::totally_ordered<fbvector<T, Allocator>> {
 
   static void
   S_uninitialized_copy_bits(T* dest, const T* first, const T* last) {
-    std::memcpy((void*)dest, (void*)first, (last - first) * sizeof(T));
+    if (last != first) {
+      std::memcpy((void*)dest, (void*)first, (last - first) * sizeof(T));
+    }
   }
 
   static void
   S_uninitialized_copy_bits(T* dest, std::move_iterator<T*> first,
                        std::move_iterator<T*> last) {
     T* bFirst = first.base();
     T* bLast = last.base();
-    std::memcpy((void*)dest, (void*)bFirst, (bLast - bFirst) * sizeof(T));
+    if (bLast != bFirst) {
+      std::memcpy((void*)dest, (void*)bFirst, (bLast - bFirst) * sizeof(T));
+    }
   }
 
   template <typename It>