boot: cover BCD offsets past the buffer

Add a zero-length test for offset > max so the first bounds guard
is covered separately from the length overflow check.

Follow-up: 231857cfe8

src/boot/test-bcd.c

@@ -96,6 +96,7 @@ TEST(offset_bounds) {
         assert_se(!BAD_ARRAY(Key, key_name, 0, 0, sizeof(Key)));
 
         assert_se(bad_offset(0, sizeof(Key) + 1, sizeof(Key)));
+        assert_se(bad_offset(sizeof(Key) + 1, 0, sizeof(Key)));
         assert_se(bad_offset(UINT32_MAX, 2, UINT32_MAX));
 }