Fix ObjectSpace.trace_object_allocations_stop to not raise if the tracepoint were not initialized

ext/objspace/object_tracing.c

@@ -290,8 +290,12 @@ trace_object_allocations_stop(VALUE self)
     }
 
     if (arg->running == 0) {
-	rb_tracepoint_disable(arg->newobj_trace);
-	rb_tracepoint_disable(arg->freeobj_trace);
+        if (arg->newobj_trace != 0) {
+            rb_tracepoint_disable(arg->newobj_trace);
+        }
+        if (arg->freeobj_trace != 0) {
+            rb_tracepoint_disable(arg->freeobj_trace);
+        }
     }
 
     return Qnil;

test/objspace/test_objspace.rb

@@ -164,6 +164,15 @@ class TestObjSpace < Test::Unit::TestCase
     end;
   end
 
+  def test_trace_object_allocations_stop_first
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      require "objspace"
+      # Make sure stoping before the tracepoints are initialized doesn't raise. See [Bug #17020]
+      ObjectSpace.trace_object_allocations_stop
+    end;
+  end
+
   def test_trace_object_allocations
     ObjectSpace.trace_object_allocations_clear # clear object_table to get rid of erroneous detection for c0
     Class.name