Bug#14597605 Issue with Null-value user on slave

An "orthographic" typo in User_var::set_deferred() was made in fixes for
bug@14275000. While editing the signature of the initial patch to remove
the only argument, the assigned value of the argument remained in the body ... 
to be successfully compiled (!) thanks to names coincidence:
the arg to User_var method and its member.

Fixed with correcting the typo.

sql/log_event.h

@@ -2508,7 +2508,7 @@ class User_var_log_event: public Log_event
      and which case the applier adjusts execution path.
   */
   bool is_deferred() { return deferred; }
-  void set_deferred() { deferred= val; }
+  void set_deferred() { deferred= true; }
 #endif
   bool is_valid() const { return 1; }