Fix assorted resource leaks in new pg_createsubscriber code.

Fix assorted resource leaks in new pg_createsubscriber code.

Various error paths did not release resources before returning.
While it's likely that the program would just exit shortly later,
none of the functions in question have summary exit(1) calls,
so they should not be assuming that.

Ranier Vilela and Tom Lane, per reports from Coverity

Discussion: https://postgr.es/m/CAEudQAr2_SZFxB4kXJiL4+2UaNZxUk5UBJtj0oXyJYMGZu-03g@mail.gmail.com