[ODE] Patch for a missing assert in mass.cpp

[Remi Ricard]

Hi,

This is a simple patch to add a assert in one of the function. Other 
function already have this assert.I submitted the patch on the 
sourceforge website.

I'm only posting this here to let the developer with cvs acces know that 
there is a new patch.

This is a patch against the latest UNSTABLE code (checked out the 
14/02/2006).

The patch is produced with:
diff -b -c3 old_file.cpp my_dir/new_file.cpp > file.patch

Description:
Add a assert as the other function to check if the axis is valid


Remi