pgsql: Fix error when trying to delete page with half-dead left sibling

Fix error when trying to delete page with half-dead left sibling.

The new page deletion code didn't cope with the case the target page's
right sibling was marked half-dead. It failed a sanity check which checked
that the downlinks in the parent page match the lower level, because a
half-dead page has no downlink. To cope, check for that condition, and
just give up on the deletion if it happens. The vacuum will finish the
deletion of the half-dead page when it gets there, and on the next vacuum
after that the empty can be deleted.

Reported by Jeff Janes.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8da31837803e95f26642019d859c3309d1c125fa

Modified Files
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src/backend/access/nbtree/nbtpage.c | 65 ++++++++++++++++++++++++++++++++---
1 file changed, 61 insertions(+), 4 deletions(-)