transcode.c: GC guard

* transcode.c (rb_econv_substr_append): prevent src from GC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42851 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

transcode.c

@@ -1850,6 +1850,7 @@ rb_econv_substr_append(rb_econv_t *ec, VALUE src, long off, long len, VALUE dst,
         rb_econv_check_error(ec);
     }
 
+    RB_GC_GUARD(src);
     return dst;
 }