refix r42525 set stdio_file only if stdio

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42527 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/readline/readline.c

@@ -503,7 +503,8 @@ readline_s_set_input(VALUE self, VALUE input)
             rb_sys_fail("fdopen");
         }
         rl_instream = f;
-        ifp->stdio_file = f;
+        if (f == stdin)
+            ifp->stdio_file = f;
         readline_instream = input;
     }
     return input;
@@ -558,7 +559,8 @@ readline_s_set_output(VALUE self, VALUE output)
             rb_sys_fail("fdopen");
         }
         rl_outstream = f;
-        ofp->stdio_file = f;
+        if (f == stdout)
+            ofp->stdio_file = f;
         readline_outstream = output;
     }
     return output;