wimax: wimax_msg_alloc() returns ERR_PTR not null

wimax_msg_alloc() returns an ERR_PTR and not null.  I changed it to test
for ERR_PTR instead of null.  I also added a check in front of the
kfree() because kfree() can handle null but not ERR_PTR.

Signed-off-by: Dan Carpenter <error27@gmail.com>