segfault in grompp with pulling and free-energy (if foreign_lambda empty)
With pulling and free_energy=yes, grompp gives as segfault if mdp option
is empty. If
foreign_lambda = 0 (for example),
no error occurs. Because foreign_lambda is empty by default, this error is likely to occur.
Fixes redmine issue #1071
Fixes issue where pull code can crash grompp if foreign lambda
is not defined in the mdp. Some simplification of code path for
initialization of lambda as a side product.