fix division by zero error in linear presolve

Author: michaelbynum

pyomo/repn/plugins/nl_writer.py

@@ -1785,7 +1785,8 @@ def _linear_presolve(self, comp_by_linear_var, lcon_by_linear_nnz, var_bounds):
                 a = -coef2 / coef
                 x = id2
                 b = expr_info.const = (lb - expr_info.const) / coef
-                expr_info.linear[x] = a
+                if a != 0:
+                    expr_info.linear[x] = a
                 substitutions_by_linear_var[x].add(_id)
                 eliminated_vars[_id] = expr_info
                 logger.debug(