rlc v2: some fixes for 'bugs' found by coverity scan

Minor fixes, doesn't change anything. Not sure these are 'bugs' either,
but let's be polite with coverity scan...

One thing was not changed.
Coverity scan says:
*** CID 357991:  Memory - illegal accesses  (USE_AFTER_FREE)
/home/carabe/raphael/openairinterface5g/openair2/LAYER2/rlc_v2/rlc_entity_am.c: 507 in tx_list_remove_sn()
501         } else {
502           prev = cur;
503           cur = cur->next;
504         }
505       }
506
>>>     CID 357991:  Memory - illegal accesses  (USE_AFTER_FREE)
>>>     Using freed pointer "head.next".
507       return head.next;
508     }
509
510     void cleanup_sdu_list(rlc_entity_am_t *entity)
511     {
512       rlc_sdu_t head;

But as far as I understand, there is no problem. We don't access
head.next at all if it has been freed. Or is there some aliasing
going on there (pointer aliasing)? I doubt it. False positive?