Fix ReadBuffer() to correctly handle the case where it's trying to extend

the relation but it finds a pre-existing valid buffer.  The buffer does not
correspond to any page known to the kernel, so we *must* do smgrextend to
ensure that the space becomes allocated.  The 7.x branches all do this
correctly, but the corner case got lost somewhere during 8.0 bufmgr rewrites.
(My fault no doubt :-( ... I think I assumed that such a buffer must be
not-BM_VALID, which is not so.)