Fix edge-case in identify 'end' upper-bound

Just my luck, this was found immediately in the CI, right when I
uploaded the project thinking I was done.

Thankfully the fix is easy and (in hindsight) quite obvious.

src/include/interval-map/interval-map.hh

@@ -35,7 +35,7 @@ public:
         if (insert_begin)
             it = underlying_.insert(it, {begin, val});
         // Get the proper upper-bound for `end`
-        it = (it == underlying_.end()) ? it : std::next(it);
+        it = (it == underlying_.end() || end < it->first) ? it : std::next(it);
         if (!(at_upper_bound(it) == end_val))
             underlying_.insert(it, {end, end_val});
     }