regress: fix robust regression

The bisection always terminated after one iteration. Change the code to
check if the middle is different from the lower and upper limits as
suggested in the original recipe.

This fixes commit b14689d59b.

regress.c

@@ -594,6 +594,8 @@ RGR_FindBestRobustRegression
     /* OK, so the root for b lies in (blo, bhi). Start bisecting */
     do {
       bmid = 0.5 * (blo + bhi);
+      if (!(blo < bmid && bmid < bhi))
+        break;
       eval_robust_residual(x + start, y + start, n_points, bmid, &a, &rmid);
       if (rmid == 0.0) {
         break;
@@ -606,7 +608,7 @@ RGR_FindBestRobustRegression
       } else {
         assert(0);
       }
-    } while ((bhi - blo) > tol && (bmid - blo) * (bhi - bmid) > 0.0);
+    } while (bhi - blo > tol);
 
     *b0 = a;
     *b1 = bmid;