Explicitly cast literal 0 to the expected bound_t type because K&R C is
awful.

--- unzip60/extract.c	2020-06-13 22:51:33.799778287 -0500
+++ unzip60/extract.c	2020-06-14 00:00:50.669719055 -0500
@@ -520,7 +520,7 @@ int extract_or_test_files(__G)    /* ret
         return PK_MEM;
     }
     if ((G.extra_bytes != 0 &&
-         cover_add((cover_t *)G.cover, 0, G.extra_bytes) != 0) ||
+         cover_add((cover_t *)G.cover, (bound_t)0, G.extra_bytes) != 0) ||
         (G.ecrec.have_ecr64 &&
          cover_add((cover_t *)G.cover, G.ecrec.ec64_start,
                    G.ecrec.ec64_end) != 0) ||