pstore/ram: Fix error handling during przs allocation

persistent_ram_new() returns ERR_PTR() value on errors, so during
freeing of the przs we should check for both NULL and IS_ERR() entries,
otherwise bad things will happen.
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/pstore/ram.c

@@ -260,7 +260,7 @@ static void ramoops_free_przs(struct ramoops_context *cxt)
 	if (!cxt->przs)
 		return;
 
-	for (i = 0; cxt->przs[i]; i++)
+	for (i = 0; !IS_ERR_OR_NULL(cxt->przs[i]); i++)
 		persistent_ram_free(cxt->przs[i]);
 	kfree(cxt->przs);
 }