flux-fsck: fix potential segfault

Problem: A "missing_indexes" list is not created unless it is
needed.  Under some rare error scenarios, this list could be
accessed without being created.

Check that the list is created before using it.

Author: chu11

src/cmd/builtin/fsck.c

@@ -357,6 +357,7 @@ static void fsck_valref (struct fsck_ctx *ctx,
 
         /* can only recover if errors were all bad references */
         if (ctx->repair
+            && fvd.missing_indexes
             && fvd.errorcount == zlist_size (fvd.missing_indexes)) {
             json_t *repaired = repair_valref (ctx, treeobj, &fvd);