ext/coverage/coverage.c: Fix the condition for non-experimental mode

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59875 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/coverage/coverage.c

@@ -30,7 +30,7 @@ rb_coverage_start(int argc, VALUE *argv, VALUE klass)
         if (!e || !*e) experimental_mode_enabled = 0;
     }
 
-    if (!experimental_mode_enabled)
+    if (!experimental_mode_enabled && argc != 0)
 	rb_error_arity(argc, 0, 0);
     rb_scan_args(argc, argv, "01", &opt);