refs.c: abort ref search if ref array is empty

refs.c: abort ref search if ref array is empty

The bsearch() implementation on IRIX 6.5 segfaults if it is passed NULL
for the base array argument even if number-of-elements is zero.  So, let's
work around it by detecting an empty array and aborting early.

This is a useful optimization in its own right anyway, since we avoid a
useless allocation and initialization of the ref_entry when the ref array
is empty.

Signed-off-by: Brandon Casey <redacted>
Signed-off-by: Junio C Hamano <redacted>