There was a bug in describe_statement. If executing `DESC FUNCTION <uda name>` or ` DESC AGGREGATE <udf name>`, Scylla was crashing because the function was found (`functions::find()` searches both UDFs and UDAs) but the function was bad and the pointer wasn't checked after cast.
Added a test for this.
Fixes: #14360Closes#14332
* github.com:scylladb/scylladb:
cql-pytest:test_describe: add test for filtering UDF and UDA
cql3:statements:describe_statement: check pointer to UDF/UDA