folly: fbvector: ubsan: avoid memcpy(dest, nullptr, 0)

Reviewed By: meyering

Differential Revision: D3277138

fbshipit-source-id: 7050d18c256ac15c8122d4af59aa31208b1466fd

folly/FBVector.h

@@ -636,8 +636,10 @@ private:
   }
 
   void relocate_move_or_memcpy(T* dest, T* first, T* last, std::true_type) {
+    if (first != nullptr) {
       std::memcpy((void*)dest, (void*)first, (last - first) * sizeof(T));
     }
+  }
 
   void relocate_move_or_memcpy(T* dest, T* first, T* last, std::false_type) {
     relocate_move_or_copy(dest, first, last, relocate_use_move());