MXS-1740 Hintfilter leaks memory

Single spot where an existing hint ptr was overwritten. Removed gwbuf_add_hint()
because it was  adding hints at the opposite end compared to functions in hint.h.
Added hint_splice() to replace.

server/core/buffer.cc

@@ -814,28 +814,6 @@ gwbuf_make_contiguous(GWBUF *orig)
     return newbuf;
 }
 
-void
-gwbuf_add_hint(GWBUF *buf, HINT *hint)
-{
-    HINT *ptr;
-
-    spinlock_acquire(&buf->gwbuf_lock);
-    if (buf->hint)
-    {
-        ptr = buf->hint;
-        while (ptr->next)
-        {
-            ptr = ptr->next;
-        }
-        ptr->next = hint;
-    }
-    else
-    {
-        buf->hint = hint;
-    }
-    spinlock_release(&buf->gwbuf_lock);
-}
-
 size_t gwbuf_copy_data(const GWBUF *buffer, size_t offset, size_t bytes, uint8_t* dest)
 {
     uint32_t buflen;

server/core/hint.cc

@@ -113,6 +113,32 @@ hint_create_route(HINT *head, HINT_TYPE type, const char *data)
     return hint;
 }
 
+
+/**
+ * Insert a hint list before head.
+ *
+ * @param head  Element before which contents is inserted.
+ *              May be NULL, in which case the result is list.
+ * @param list  Hint list to prepend
+ * @return Head of list
+ */
+HINT *
+hint_splice(HINT *head, HINT *list)
+{
+    ss_dassert(list);
+    if (head)
+    {
+        HINT* tail = list;
+        while (tail->next)
+        {
+            tail = tail->next;
+        }
+        tail->next = head;
+    }
+
+    return list;
+}
+
 /**
  * Create name/value parameter hint
  *

server/core/test/test_buffer.cc

@@ -467,10 +467,6 @@ test1()
     ss_info_dassert(size == buflen, "Incorrect buffer size");
     ss_info_dassert(0 == GWBUF_EMPTY(buffer), "Buffer should not be empty");
     ss_info_dassert(GWBUF_IS_TYPE_UNDEFINED(buffer), "Buffer type should be undefined");
-    ss_dfprintf(stderr, "\t..done\nSet a hint for the buffer");
-    hint = hint_create_parameter(NULL, (char*)"name", (char*)"value");
-    gwbuf_add_hint(buffer, hint);
-    ss_info_dassert(hint == buffer->hint, "Buffer should point to first and only hint");
     ss_dfprintf(stderr, "\t..done\nSet a property for the buffer");
     gwbuf_add_property(buffer, (char*)"name", (char*)"value");
     ss_info_dassert(0 == strcmp("value", gwbuf_get_property(buffer, (char*)"name")),