Switch Get[Local]BufferDescriptor() to use a signed value in input

Switch Get[Local]BufferDescriptor() to use a signed value in input

GetBufferDescriptor() and GetLocalBufferDescriptor() took a uint32
buffer index, but every real caller derives the index from a Buffer:
- Unsigned value for shared buffers.
- Signed value for local buffers.

Both routines now take in input a signed number, GetBufferDescriptor()
gaining an assertion checking that the input value is in the range
allowed by the GUC shared_buffers.  This work is a follow-up of
e18b0cb7344c, where we found that passing down a value for a local
buffer was undetected and finished outside the range of NBuffers.

While monitoring all the existing callers of *BufferDescriptor(), the
only consumer that passes does an unsigned value is ClockSweepTick(),
whose result is always a module of NBuffers.

Suggested-by: Andres Freund <andres@anarazel.de>
Author: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAExHW5uzRMYVZsXXS3HXXT0fG_sNrpUhUqwP4NorhaCqH9JDhA@mail.gmail.com