find_reference_location(): make function safe for empty snapshots

find_reference_location(): make function safe for empty snapshots

This function had two problems if called for an empty snapshot (i.e.,
`snapshot->start == snapshot->eof == NULL`):

* It checked `NULL < NULL`, which is undefined by C (albeit highly
  unlikely to fail in the real world).

* (Assuming the above comparison behaved as expected), it returned
  NULL when `mustexist` was false, contrary to its docstring.

Change the check and fix the docstring.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>