gossmap: don't crash if we hit a zero-length record.

We have a report of this happening under ZFS.  We cannot do much if
this really is a problem where we can't read back what we write, but
this avoids the immediate crash.

Fixes: ElementsProject#7971
Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

common/gossmap.c

@@ -701,6 +701,11 @@ static bool map_catchup(struct gossmap *map, bool *changed)
 		if (map->map_end + reclen > map->map_size)
 			break;
 
+		/* Under zfs, apparently we can see zeroes here.  I'm hoping this effect
+		 * is transient: hope to make progress next time */
+		if (be16_to_cpu(ghdr.len) < sizeof(be16))
+			break;
+
 		off = map->map_end + sizeof(ghdr);
 		type = map_be16(map, off);
 		if (type == WIRE_CHANNEL_ANNOUNCEMENT) {