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 82467f627b.  No backpatch is required, as this is new to
v19.

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

src/backend/storage/buffer/bufmgr.c

@@ -5831,8 +5831,6 @@ MarkBufferDirtyHint(Buffer buffer, bool buffer_std)
 {
 	BufferDesc *bufHdr;
 
-	bufHdr = GetBufferDescriptor(buffer - 1);
-
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "bad buffer ID: %d", buffer);
 
@@ -5842,6 +5840,8 @@ MarkBufferDirtyHint(Buffer buffer, bool buffer_std)
 		return;
 	}
 
+	bufHdr = GetBufferDescriptor(buffer - 1);
+
 	MarkSharedBufferDirtyHint(buffer, bufHdr,
 							  pg_atomic_read_u64(&bufHdr->state),
 							  buffer_std);