lib: misc static analysis fixes

This fixes a few warnings from coverity, none of which really
interesting.

src/libostree/ostree-repo-commit.c

@@ -1683,8 +1683,8 @@ ostree_repo_prepare_transaction (OstreeRepo     *self,
   /* Set up to abort the transaction if we return early from this function.
    * This needs to be manually built here due to a circular dependency. */
   g_autoptr(OstreeRepoAutoTransaction) txn = g_malloc(sizeof(OstreeRepoAutoTransaction));
+  g_assert (txn != NULL);
   txn->repo = self;
-  (void) txn; /* Add use to silence static analysis */
 
   memset (&self->txn.stats, 0, sizeof (OstreeRepoTransactionStats));