should not free local hook_list here.

exec_hooks_postcheck() clean executed hook_list if it is needed.
list_exec is freed if there are no events and this list is local
event (connected to specific iseq). However, iseq keeps to point
this local hook_list, freed list. To prevent this situation,
do not free hook_list here even if it has no events.

This issue is reported by @joker1007.
https://twitter.com/joker1007/status/1153649170797830144

test/ruby/test_settracefunc.rb

@@ -2166,4 +2166,21 @@ class TestSetTraceFunc < Test::Unit::TestCase
     tp.enable {obj.example}
     assert ok, "return event should be emitted"
   end
+
+  def test_disable_local_tracepoint_in_trace
+    assert_normal_exit <<-EOS
+    def foo
+      trace = TracePoint.new(:b_return){|tp|
+        tp.disable
+      }
+      trace.enable(target: method(:bar))
+    end
+    def bar
+      100.times{|i|
+        foo; foo
+      }
+    end
+    bar
+    EOS
+  end
 end

vm_trace.c

@@ -198,9 +198,6 @@ clean_hooks(const rb_execution_context_t *ec, rb_hook_list_t *list)
     }
     else {
         /* local events */
-        if (list->events == 0) {
-            ruby_xfree(list);
-        }
     }
 }