contiguous_iterator: do not call snapshot_range for default-constructed iterator

Origianl implementation resulted in the following array_iterator_noconv_0_test
failure under UBSAN:

"runtime error: applying non-zero offset 4 to null pointer"

Author: igchor

include/libpmemobj++/experimental/contiguous_iterator.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright 2018-2020, Intel Corporation
+ * Copyright 2018-2021, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -245,7 +245,7 @@ struct range_snapshotting_iterator
 	{
 		assert(data <= ptr);
 
-		if (snapshot_size > 0)
+		if (snapshot_size && ptr)
 			snapshot_range(ptr);
 	}