core: fix unlikely memory leak in completion

ChangeLog.adoc

@@ -28,6 +28,7 @@ New features::
 
 Bug fixes::
 
+  * core: fix memory leak in completion
   * core: flush stdout/stderr before forking in hook_process function (issue #1441)
   * core: fix evaluation of condition with nested "if" (issue #1434)
   * irc: fix crash when receiving a malformed message 324 (channel mode)

src/gui/gui-completion.c

@@ -1320,6 +1320,8 @@ gui_completion_search (struct t_gui_completion *completion, int direction,
     {
         case GUI_COMPLETION_NULL:
             /* should never be executed */
+            if (old_word_found)
+                free (old_word_found);
             return;
         case GUI_COMPLETION_COMMAND:
             gui_completion_command (completion);