pgsql: Avoid double free of _SPI_current->tuptable.

Log Message:
-----------
Avoid double free of _SPI_current->tuptable.  AtEOSubXact_SPI() now tries to
release it in a subtransaction abort, but this neglects possibility that
someone outside SPI already did.  Fix is for spi.c to forget about a tuptable
as soon as it's handed it back to the caller.
Per bug #2817 from Michael Andreen.

Modified Files:
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    pgsql/src/backend/executor:
        spi.c (r1.165 -> r1.166)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c.diff?r1=1.165&r2=1.166)