bootspec: fix null-dereference-read

Fixes [oss-fuzz#53578](https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=53578).
Fixes #25450.

src/shared/bootspec.c

@@ -980,6 +980,8 @@ static int boot_config_find(const BootConfig *config, const char *id) {
         if (id[0] == '@') {
                 if (!strcaseeq(id, "@saved"))
                         return -1;
+                if (!config->entry_selected)
+                        return -1;
                 id = config->entry_selected;
         }
 

test/fuzz/fuzz-bootspec/clusterfuzz-testcase-minimized-fuzz-bootspec-5731869371269120

@@ -0,0 +1 @@
+{"config":"default @saved","loader":[""]}