fix for crash with adjoint method when forward integration fails
fix for crash with adjoint method when forward integration fails

AMICI-developer/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI