pktbuf: assert for potential null pointer

In practice, this should not happen, since `_first_unused` should be
already set, if `_too_small_hole()` returns true.

sys/net/crosslayer/ng_pktbuf_static/ng_pktbuf_static.c

@@ -429,6 +429,8 @@ static inline bool _too_small_hole(_unused_t *a, _unused_t *b)
 
 static inline _unused_t *_merge(_unused_t *a, _unused_t *b)
 {
+    assert(b != NULL);
+
     a->next = b->next;
     a->size = b->size + ((uint8_t *)b - (uint8_t *)a);
     return a;