fix warning: dereference of a null pointer

This fixes warning: Access to field 'next' results in a
dereference of a null pointer (loaded from variable 'prev')
[clang-analyzer-core.NullDereference]

Bug: None
Test: The warning is gone.
Change-Id: I7a839f94b9c6d56a91bfda1097ba23ff247c9db1