ovs-ofctl: Avoid impossible check for !osm in fetch_port_by_stats().

At the time of the call to ofpbuf_at(), we know that the ofp_stats_msg is
present because ofputil_decode_msg_type() reported that it was.  Therefore,
we can use ofpbuf_at_assert() and don't have to check for a null pointer.

Found by clang.

Signed-off-by: Ben Pfaff <blp@nicira.com>