mac80211: handle allocation failures in mesh_pathtbl_init()

The calls to kzalloc() weren't checked here and it upsets the static
checkers.  Obviously they're not super likely to fail, but we might
as well add some error handling.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>