iwlagn: fix panic in iwl{5000,4965}_rx_reply_tx

In some cases firmware can give us bad value of index in transmit
buffers array. This patch add sanity check for such values and return
from processing function instantly when it happens.

https://bugzilla.redhat.com/show_bug.cgi?id=521931

Patch was tested by reporter on iwl5000. I think check can be also
helpful for 4965.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>