PL/Python: Fix potential NULL pointer dereference

PL/Python: Fix potential NULL pointer dereference

After d0aa965c0a0ac2ff7906ae1b1dad50a7952efa56, one error path in
PLy_spi_execute_fetch_result() could result in the variable "result"
being dereferenced after being set to NULL.  To fix that, just clear the
resources right there and return early.

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