[ODE] memory leak from redundant allocation in dBodyCreate

[Riley Lark]

I would fix this myself, but I am extremely wary of my SVN skills:

in dBodyCreate on line 254 of ode.cpp, dBodySetAutoDisableDefaults is
called.  This eventually gets to line 909 and 910 of ode.cpp which, if
adis.average_samples > 0, allocate space for the samples.

Then control returns to dBodyCreate which, on line 261 of ode.cpp,
allocates space for the samples again.

I fixed this error by checking to make sure average_lvel_buffer and
average_avel_buffer are still NULL just before allocating them in
dBodyCreate.