pgsql: Fix MarkBufferDirtyHint() to not call GetBufferDescriptor() for

Fix MarkBufferDirtyHint() to not call GetBufferDescriptor() for local buffers

GetBufferDescriptor() was called before checking if the buffer is local.
Such buffers have a negative ID, meaning that we could call
GetBufferDescriptor() with a wrapped-around uint32 value causing a
potential out-of-bound access to the BufferDescriptors array.

This is harmless in the existing code for the current uses of
MarkBufferDirtyHint(), but the author has found a way to make that
buggy while working on a different patch set, and the order of the
operations is wrong.

Oversight in 82467f627bd4. No backpatch is required, as this is new to
v19.

Author: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAExHW5uzRMYVZsXXS3HXXT0fG_sNrpUhUqwP4NorhaCqH9JDhA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e18b0cb7344cb4bd28468f6c0aeeb9b9241d30aa

Modified Files
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src/backend/storage/buffer/bufmgr.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)