MXS-2470 Invalidate tail pointers

Although not made unambiguously clear, it is *ONLY* allowed to
manipulate GWBUFs that are the first link of a GWBUF chain (that
may consist of just one link).

With this change, in debug mode the tail pointer of intermediate
links is invalidated so that an attempt to use an intermediate
link is likely to cause a crash.

server/core/buffer.cc

@@ -29,6 +29,23 @@ static void             gwbuf_free_one(GWBUF* buf);
 static buffer_object_t* gwbuf_remove_buffer_object(GWBUF* buf,
                                                    buffer_object_t* bufobj);
 
+#if defined(SS_DEBUG)
+inline void invalidate_tail_pointers(GWBUF* head)
+{
+    if (head && head->next)
+    {
+        GWBUF* link = head->next;
+        while (link != head->tail)
+        {
+            link->tail = reinterpret_cast<GWBUF*>(0xdeadbeef);
+            link = link->next;
+        }
+    }
+}
+#else
+inline void invalidate_tail_pointers(GWBUF* head) {}
+#endif
+
 /**
  * Allocate a new gateway buffer structure of size bytes.
  *
@@ -218,6 +235,8 @@ GWBUF* gwbuf_clone(GWBUF* buf)
         {
             rval->tail = clonebuf;
         }
+
+        invalidate_tail_pointers(rval);
     }
 
     return rval;
@@ -333,6 +352,9 @@ GWBUF* gwbuf_split(GWBUF** buf, size_t length)
         }
 
         *buf = buffer;
+
+        invalidate_tail_pointers(*buf);
+        invalidate_tail_pointers(head);
     }
 
     return head;
@@ -475,6 +497,8 @@ GWBUF* gwbuf_append(GWBUF* head, GWBUF* tail)
     head->tail->next = tail;
     head->tail = tail->tail;
 
+    invalidate_tail_pointers(head);
+
     return head;
 }
 
@@ -500,6 +524,8 @@ GWBUF* gwbuf_consume(GWBUF* head, unsigned int length)
         }
     }
 
+    invalidate_tail_pointers(head);
+
     mxb_assert(head == NULL || (head->end >= head->start));
     return head;
 }