We were never doing clearerr() on the output stream, which results in a
message being printed after each result once an EOF is seen:

could not print result table: Success

This message was added by commit b0343699 (in the pg13 era); before
that, the error indicator would never be examined.  So backpatch only
that far back, even though the actual bug (to wit: the fact that the
error indicator is never cleared) is older.