pgsql: Fix two off-by-one errors in bufmgr.c.

Fix two off-by-one errors in bufmgr.c.

In 4b4b680c I passed a buffer index number (starting from 0) instead of
a proper Buffer id (which start from 1 for shared buffers) in two
places.

This wasn't noticed so far as one of those locations isn't compiled at
all (PrintPinnedBufs) and the other one (InvalidBuffer) requires a
unlikely, but possible, set of circumstances to trigger a symptom.

To reduce the likelihood of such incidents a bit also convert existing
open coded mappings from buffer descriptors to buffer ids with
BufferDescriptorGetBuffer().

Author: Qingqing Zhou
Reported-By: Qingqing Zhou
Discussion: CAJjS0u2ai9ooUisKtkV8cuVUtEkMTsbK8c7juNAjv8K11zeCQg@mail.gmail.com
Backpatch: 9.5 where the private ref count infrastructure was introduced

Branch
------
REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/43a8ed26c97e36d971b6018d1bc94ad5c52d169b

Modified Files
--------------
src/backend/storage/buffer/bufmgr.c |   36 ++++++++++++++++++-----------------
1 file changed, 19 insertions(+), 17 deletions(-)