ovs_stats: fix potential NULL-string dereference

The YAJL_GET_STRING macro may return NULL; ensure that the returned
string is non-NULL, before passing it to ovs_stats_new_port(), within
the context of the ovs_stats_update_bridge() function.

Since this change introduces another return path, a single 'failure'
return point is introduced to ovs_stats_bridge().

Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>