rebase: change assertion to avoid

It looks like we're getting the operation and not doing anything
with it, when in fact we are asserting that it's not null.  Simply
assert that we are within the operation boundary instead of using
the `git_array_get` macro to do this for us.

src/rebase.c

@@ -1047,15 +1047,12 @@ static int rebase_commit_inmemory(
 	const char *message_encoding,
 	const char *message)
 {
-	git_rebase_operation *operation;
 	git_commit *commit = NULL;
 	int error = 0;
 
-	operation = git_array_get(rebase->operations, rebase->current);
-
-	assert(operation);
 	assert(rebase->index);
 	assert(rebase->last_commit);
+	assert(rebase->current < rebase->operations.size);
 
 	if ((error = rebase_commit__create(&commit, rebase, rebase->index,
 		rebase->last_commit, author, committer, message_encoding, message)) < 0)