Fix build by silencing zero-length memset warning.

runtime/vm/zone.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ZONE_H_
 #define VM_ZONE_H_
 
 #include "platform/utils.h"
 #include "vm/allocation.h"
 #include "vm/handles.h"
@@ skipping 100 matching lines @@
   }
 
   // Delete all objects and free all memory allocated in the zone.
   void DeleteAll();
 
   // Does not actually free any memory. Enables templated containers like
   // BaseGrowableArray to use different allocators.
   template <class ElementType>
   void Free(ElementType* old_array, intptr_t len) {
 #ifdef DEBUG
-    memset(old_array, kZapUninitializedByte, len * sizeof(ElementType));
+    if (len > 0) {
+      memset(old_array, kZapUninitializedByte, len * sizeof(ElementType));
+    }
 #endif
   }
 
 #if defined(DEBUG)
   // Dump the current allocated sizes in the zone object.
   void DumpZoneSizes();
 #endif
 
   // Overflow check (FATAL) for array length.
   template <class ElementType>
@@ skipping 151 matching lines @@
     memmove(reinterpret_cast<void*>(new_data),
             reinterpret_cast<void*>(old_data),
             old_len * kElementSize);
   }
   return new_data;
 }
 
 }  // namespace dart
 
 #endif  // VM_ZONE_H_