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: NBrandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

refs.c

@@ -110,6 +110,9 @@ static struct ref_entry *search_ref_array(struct ref_array *array, const char *n
 	if (name == NULL)
 		return NULL;
 
+	if (!array->nr)
+		return NULL;
+
 	len = strlen(name) + 1;
 	e = xmalloc(sizeof(struct ref_entry) + len);
 	memcpy(e->name, name, len);