empty query string is passed to PQexecParams and related functions.  Its
handling of the NoData response to Describe messages was subtly incorrect.
Per my report of yesterday.

Although I consider this a bug, it's a behavioral change that might affect
applications, so not back-patched.

In passing fix a second issue in the same code: it didn't react well to an
out-of-memory failure while trying to make the PGresult object.