IEEE80211-radio: Fix divide by zero

IEEE80211-radio: Fix divide by zero

Perform sanity check on parsed data rate. Data rate of zero results in a
division by zero in calculate_llac_duration. Do not use data rate if zero.

Bug: 13706
Change-Id: I7883e1052a0965f7927d3a478c0aa992693f7fe1
Reviewed-on: https://code.wireshark.org/review/21626
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Guy Harris <guy@alum.mit.edu>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>