slist/dlist: container node can't be NULL in *_PEEK_NEXT_CONTAINER

Using MPU enabled HW it was evident that a NULL access
(with offset) was happening in the TCP stack due to the
following message:
***** MPU FAULT *****
  Executing thread ID (thread): 0x20009b0c
  Faulting instruction address:  0x8034496
  Data Access Violation
  Address: 0x34
Fatal fault in essential thread! Spinning...

Turns out we are referencing a potentially de-referenced
NULL pointer in the SYS_SLIST_PEEK_NEXT_CONTAINER macro.

Let's avoid this by checking the container node for NULL.

Also fix dlist.h SYS_DLIST_PEEK_NEXT_CONTAINER with the same
issue.

Change-Id: I2e765b9af7bcaf8fb13f7c9b7e081f9e6d4928f2
Signed-off-by: Michael Scott <michael.scott@linaro.org>