After d0aa965c, one error path in
PLy_spi_execute_fetch_result() could result in the variable "result"
being dereferenced after being set to NULL.  Rearrange the code a bit to
fix that.

Also add another SPI_freetuptable() call so that that is cleared in all
error paths.

discovered by John Naylor <jcnaylor@gmail.com> via scan-build

ideas and review by Tom Lane