Fix C++: Ensure diagnostics variables in ReturnData are always of expected length (#1438)
Fix C++: Ensure diagnostics variables in ReturnData are always of expected length (#1438)

Closes #1401
AMICI-developer/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,FFroehlich/AMICI,AMICI-developer/AMICI,AMICI-developer/AMICI,FFroehlich/AMICI