Added assertion checking if next element on list is valid. Change-Id: I9f4d969dc84e5fbee9d72d764f57fbd9480ab197 Signed-off-by: Wojciech Malikowski <wojciech.malikowski@intel.com> Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/456774 Tested-by: SPDK CI Jenkins <sys_sgci@intel.com> Reviewed-by: Jim Harris <james.r.harris@intel.com> Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>