Make addLJType work when the LENNARD_JONES_CCOEF array is present.  Currently,
ParmEd spits out a *broken* topology file (as in, causes segfault or other
crashes) when you use

add12_6_4 <blah>
addLJType <blah>

However, the order

addLJType <blah>
add12_6_4 <blah>

works fine.  With this commit, both sets of command produce an identical
prmtop file.