Unsafe access BufferDescriptors array in BufferGetLSNAtomic()

Hi,

While learning gist index insert codes, I find a little issue with
BufferGetLSNAtomic().
At first, it wants to get bufHdr by accessing the buffer descriptor array,
as below:

BufferDesc *bufHdr = GetBufferDescriptor(buffer - 1);

However, it doesn't check whether the passed buffer is a local or shared
buffer.
If the buffer is local, then buffer < 0; it will be cast to uint32 when
passed to GetBufferDescriptor().
This may be unsafe, although no someone reports the problem.

I tweak a few codes; see the attached patch.
Any thoughts?

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Thanks,
Tender Wang