Completion of error handling

Would you like to [add more error handling](https://github.com/citusdata/citus/blob/a9652daef76655847881456bd7e0ad1da17e6804/src/backend/distributed/planner/multi_join_order.c#L1111) for [return values from functions like "strdup"](http://pubs.opengroup.org/onlinepubs/9699919799/functions/strdup.html)?

Start in src/backend/distributed/planner/multi_join_order.c at the linked location, then review nearby calls that return values like strdup and consult the linked strdup documentation. Identify unchecked failure paths and determine how error handling should behave consistently; done means the relevant return values are handled without leaving the planner in an invalid state.

Written by the indexing model from the issue text.