bootctl: use boot_config_default_entry() where appropriate

src/shared/bootspec.h

@@ -56,6 +56,7 @@ typedef struct BootConfig {
 
         BootEntry *entries;
         size_t n_entries;
+
         ssize_t default_entry;
         ssize_t selected_entry;
 
@@ -86,10 +87,12 @@ static inline BootEntry* boot_config_default_entry(BootConfig *config) {
         if (config->default_entry < 0)
                 return NULL;
 
+        assert((size_t) config->default_entry < config->n_entries);
         return config->entries + config->default_entry;
 }
 
 void boot_config_free(BootConfig *config);
+
 int boot_entries_load_config(const char *esp_path, const char *xbootldr_path, BootConfig *config);
 int boot_entries_load_config_auto(const char *override_esp_path, const char *override_xbootldr_path, BootConfig *config);
 int boot_entries_augment_from_loader(BootConfig *config, char **list, bool only_auto);