vinyl: fix dump_bandwidth init in case snap_io_rate_limit is maxed out

box.cfg{snap_io_rate_limit = 0} means that the limit is maxed out hence
we must set the dump bandwidth estimate to the default value. Instead
we set it to 0, which may resulting in invalid transaction throttling.
Fix this.

Fixes commit b646fbd9 ("vinyl: use snap_io_rate_limit for initial
dump bandwidth estimate").