handling out of memory conditions when fetching row descriptions

Hi there,

using the latest git source code, I found that libpq will let the
connection stall when getRowDescriptions breaks on an out of memory
condition. I think this should better be handled differently to allow
application code to handle such situations gracefully.

For now, I changed libpq to close the connection when this happens
(please see the attached patch). This approach works fine for me so
far.

Please inform me if there are better ways to handle this.

Thanks.

Best regards,

Isidor Zeuner