Fixed issue where looking up tag 0 fails after a delete id0

Well not really fixed, more just added an assert to make sure
lfsr_rbyd_lookup is not called with tag 0. Because our alt tags only
encode less-than-or-equal and greater-than, which can be flipped
trivially, it's not possible to encode removal of tag 0 during deletes.

Fortunately, this tag should already not exist for other pragmatic
reasons, it was just used as the initial value for traversals, where it
could cause this bug.

lfs.c

@@ -1737,6 +1737,10 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
 static int lfsr_rbyd_lookup(lfs_t *lfs, const lfsr_rbyd_t *rbyd,
         lfsr_tag_t tag, lfsr_sid_t id,
         lfsr_tag_t *tag_, lfsr_sid_t *id_, lfs_off_t *off_, lfs_size_t *size_) {
+    // tag must be non-zero! zero tags may deceptively look like they work but
+    // fail when the tree contains a deleted id0
+    LFS_ASSERT(tag != 0);
+
     // no trunk yet?
     lfs_off_t branch = rbyd->trunk;
     if (!branch) {