io.c: fix uninitialized variable

* io.c (argf_next_argv): fix uninitialized variable in skipping
  message when inplace edit without backup.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47309 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

io.c

@@ -7903,8 +7903,9 @@ argf_next_argv(VALUE argf)
 			rb_io_close(rb_stdout);
 		    }
 		    fstat(fr, &st);
+		    str = ARGF.filename;
 		    if (*ARGF.inplace) {
-			str = rb_str_dup(ARGF.filename);
+			str = rb_str_dup(str);
 			rb_str_cat2(str, ARGF.inplace);
 			/* TODO: encoding of ARGF.inplace */
 #ifdef NO_SAFE_RENAME