reftable/basics: fix OOB read on binary search of empty range

`binsearch()` performs a binary search over a range of `sz` elements by
repeatedly calling the comparison function with indices into that range.
When the range is empty though, there is no valid index to call the
comparison function with. We still end up executing the comparison
function though with an index of 0, which of course will cause an
out-of-bounds read.

Return early when the range is empty.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/basics.c

@@ -152,6 +152,9 @@ size_t binsearch(size_t sz, int (*f)(size_t k, void *args), void *args)
 	size_t lo = 0;
 	size_t hi = sz;
 
+	if (!sz)
+		return 0;
+
 	/* Invariants:
 	 *
 	 *  (hi == sz) || f(hi) == true

t/unit-tests/u-reftable-basics.c

@@ -60,6 +60,17 @@ void test_reftable_basics__binsearch(void)
 	}
 }
 
+static int unreachable_lesseq(size_t i UNUSED, void *args UNUSED)
+{
+	cl_fail("comparison function called for empty range");
+	return 0;
+}
+
+void test_reftable_basics__binsearch_empty(void)
+{
+	cl_assert_equal_i(binsearch(0, &unreachable_lesseq, NULL), 0);
+}
+
 void test_reftable_basics__names_length(void)
 {
 	const char *a[] = { "a", "b", NULL };