Do not assume that we find a direction of descent

dune/tectonic/myblockproblem.hh

@@ -131,7 +131,8 @@ template <class MyConvexProblemTypeTEMPLATE> class MyBlockProblem {
     Interval<double> D;
     psi.subDiff(0, D);
     if (D[1] > 0) {
-      assert(false); // NOTE: Numerical instability can actually get us here
+      // NOTE: Numerical instability can actually get us here
+      assert(abs(D[1]) < 1e-15);
       return 0;
     }