shada: Fix non-writeable ShaDa directory handling

Before this change,

    nvim -i /etc/shada

segfaults on exit if the file does not exist and user does not have
permissions to create the file at /etc/shada.

Closes #5296
Reported in #5277
https://github.com/neovim/neovim/issues/5277#issuecomment-243937255

src/nvim/shada.c

@@ -2976,7 +2976,7 @@ shada_write_file_nomerge: {}
   if (sd_writer.cookie == NULL) {
     xfree(fname);
     xfree(tempname);
-    if (sd_reader.close != NULL) {
+    if (sd_reader.cookie != NULL) {
       sd_reader.close(&sd_reader);
     }
     return FAIL;