Fix pfree-of-already-freed-tuple when rescanning a GiST index-only scan.

Fix pfree-of-already-freed-tuple when rescanning a GiST index-only scan.

GiST's getNextNearest() function attempts to pfree the previously-returned
tuple if any (that is, scan->xs_hitup in HEAD, or scan->xs_itup in older
branches).  However, if we are rescanning a plan node after ending a
previous scan early, those tuple pointers could be pointing to garbage,
because they would be pointing into the scan's pageDataCxt or queueCxt
which has been reset.  In a debug build this reliably results in a crash,
although I think it might sometimes accidentally fail to fail in
production builds.

To fix, clear the pointer field anyplace we reset a context it might
be pointing into.  This may be overkill --- I think probably only the
queueCxt case is involved in this bug, so that resetting in gistrescan()
would be sufficient --- but dangling pointers are generally bad news,
so let's avoid them.

Another plausible answer might be to just not bother with the pfree in
getNextNearest().  The reconstructed tuples would go away anyway in the
context resets, and I'm far from convinced that freeing them a bit earlier
really saves anything meaningful.  I'll stick with the original logic in
this patch, but if we find more problems in the same area we should
consider that approach.

Per bug #14641 from Denis Smirnov.  Back-patch to 9.5 where this
logic was introduced.

Discussion: https://postgr.es/m/20170504072034.24366.57688@wrigleys.postgresql.org