babeld: Check that bodylen is within some bounds of usable

babeld: Check that bodylen is within some bounds of usable

Coverity believed that the bodylen value was read directly
from the incoming packet and then used as a loop variable.
Unfortunately it missed the fact that in babel_packet_examin
the bodylen was actually checked to ensure that it was long
enough.  So instead of checking it 2 times, generate it one
time and let coverity figure it out from that.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>