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 <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>