smalloc: zero size allocs may need to be free'd

Zero size allocations don't necessarily mean that data == NULL. So
instead check each value seperately and perform necessary operations.

src/smalloc.cc

@@ -180,14 +180,15 @@ void AllocDispose(Handle<Object> obj) {
   char* data = static_cast<char*>(obj->GetIndexedPropertiesExternalArrayData());
   size_t length = obj->GetIndexedPropertiesExternalArrayDataLength();
 
-  if (data == NULL || length == 0)
-    return;
-
+  if (data != NULL) {
     obj->SetIndexedPropertiesToExternalArrayData(NULL,
                                                  kExternalUnsignedByteArray,
                                                  0);
     free(data);
+  }
+  if (length != 0) {
     node_isolate->AdjustAmountOfExternalAllocatedMemory(-length);
+  }
 }