Avoid illegal memory access when variable name is used as a function call

The call to model_error() would return and lead to an incorrect dereferencing
of the optional<int> rv.

The call to model_error() should have been replaced by a call to error() in
db1f6c29. Then the move to an optional<int> in
46b2c735 introduced the bug.

See https://forum.dynare.org/t/preprocessing-failed-for-specific-mod-file/26705