ovs_stats: fix potential NULL dereference

ovs_stats: fix potential NULL dereference

ovs_stats_new_port() accepts a character pointer, uuid, as a parameter,
and copies it into port->uuid. Later, this value is dereferenced in
ovs_stats_update_bridge(). If uuid was NULL, then a SEGV will occur.

Resolve this issue by checking if uuid is NULL in ovs_stats_new_port().

Fixes: 481984e ("ovs_stats: Implement OVS statistics plugin.")
Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>