vinyl: vy_key_snprint: fix crash on incomplete keys
vinyl: vy_key_snprint: fix crash on incomplete keys

There may be less parts in a key than key_def->parts_count, so we should
break the loop as soon as we see part == NULL.

The crash can only be observed if vy_key_snprint is manually inserted in
the read iterator for debugging.

rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool