Fix: Invalidate ReturnData::sllh on backwards integration failure (#1327)
Fix: Invalidate ReturnData::sllh on backwards integration failure (#1327)

Closes #1325

Co-authored-by: Fabian Fröhlich <644850cb8939d255ed752957b7c4458c52900f75@schaluck.com>
AMICI-developer/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI