Deal with cases where no var_exo are specified

In mod-files without specified exogenous variables, the respective fields associated with them are not set. This leads to cryptic error messages: http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=5100

We should either disallow such mod-files or simply set

M_.maximum_exo_lag=0;
M_.maximum_exo_lead=0;