Add asserts to Fuzzy::add_point()

This commit adds asserts to ensure that the preconditions on
the points_ list are met when calling the function, namely:
- the list has at least two elements
- the first and last element are at abscissas X_MIN and X_MAX

Moreover, this commit moves the (p != points_.begin()) check
out of the if clause, and into an assert as well.

This ensures that if a bug happens here again, we will catch
it instead of sweeping it under the rug.

src/libaevol/Fuzzy.cpp

@@ -478,7 +478,14 @@ void Fuzzy::clear() {
 
 void Fuzzy::add_point(ProteinConcentration x, ProteinConcentration y)
 {
-  // Don't add a new point if there's already a point at the same x.
+  // points_ must always contain at least two elements, at X_MIN and X_MAX.
+  assert(points_.size() >= 2);
+  assert(points_.begin()->x == X_MIN);
+  assert(points_.rbegin()->x == X_MAX);
+
+  // We don't want to add a new point if there's already a point at the same x.
+  // To find out if there is such a point, we find the first point with
+  // larger x and then look at its predecessor.
   list<Point>::iterator p =
 #ifndef __OPENMP_GPU
       find_if(points_.begin(), points_.end(), [x](Point& q){return q.x > x;});
@@ -486,6 +493,11 @@ void Fuzzy::add_point(ProteinConcentration x, ProteinConcentration y)
       algorithm_cuda::find_if_point_5(points_.begin(), points_.end(), x);
 #endif
 
+  // Since there is always a point at x = X_MAX, we always find such a point,
+  // and since there is always a point at x = X_MIN (which does not
+  // match the condition), that point always has a predecessor.
+  // p should therefore never be points_.begin().
+  assert(p != points_.begin());
   if (prev(p)->x == x) {
     prev(p)->y += y;
   } else {