LibWeb: Don't skip last keyframe

Author: https://github.com/Gingeh
Commit: 4292344729
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5139
Reviewed-by: https://github.com/gmta ✅

Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -1334,8 +1334,10 @@ void StyleComputer::collect_animation_into(DOM::Element& element, Optional<CSS::
             return keyframes.begin();
         }
         auto potential_match = keyframes.find_largest_not_above_iterator(key);
-        if (output_progress.value() >= 0) {
-            return --potential_match;
+        auto next = potential_match;
+        ++next;
+        if (next.is_end()) {
+            --potential_match;
         }
         return potential_match;
     }();