Fixed problem of rows being removed during optimization, which can le…

…ad to InternalSolverErrors.

The optimizer was removing rows from the solver entirely when it should only be removing them from the list of infeasible_rows to pivot on.
Seems to be just a mistranslation from kiwi, behavior now matches kiwi.

src/solver_impl.rs

@@ -583,9 +583,8 @@ impl Solver {
     fn dual_optimise(&mut self) -> Result<(), InternalSolverError> {
         while !self.infeasible_rows.is_empty() {
             let leaving = self.infeasible_rows.pop().unwrap();
-            if let Some(mut row) = self.rows.remove(&leaving)
-                .and_then(|row| if row.constant < 0.0 { Some(row) } else { None })
-            {
+            if self.rows.get(&leaving).map_or(false, |row| row.constant < 0.0) {
+                let mut row = self.rows.remove(&leaving).unwrap();
                 let entering = self.get_dual_entering_symbol(&row);
                 if entering.type_() == SymbolType::Invalid {
                     return Err(InternalSolverError("Dual optimise failed."));