Add ensure calls to CAgg validation function

The CAgg validation logic is quite complex. This PR adds two ensure()
statements to the validation function to make it more clear (e.g., also
for static code analyzers) that we expect a valid bucket function.

tsl/src/continuous_aggs/common.c

@@ -938,8 +938,13 @@ cagg_validate_query(const Query *query, const bool finalized, const char *cagg_s
 		(bucket_info.bucket_time_offset != NULL) ||
 		(TIMESTAMP_NOT_FINITE(bucket_info.bucket_time_origin) == false);
 
+	/* At this point, we should have a valid bucket function. Otherwise, we have errored out before.
+	 */
+	Ensure(bucket_info.bucket_func != NULL, "unable to find valid bucket function");
+
 	/* Ignore time_bucket_ng in this check, since offset and origin were allowed in the past */
 	FuncInfo *func_info = ts_func_cache_get_bucketing_func(bucket_info.bucket_func->funcid);
+	Ensure(func_info != NULL, "bucket function is not found in function cache");
 	bool is_time_bucket_ng = func_info->origin == ORIGIN_TIMESCALE_EXPERIMENTAL;
 
 	/*