Bug 1611853: return early in `Selection::AddRangeAndSelectFramesAndNotifyListeners` when it didn't add a range. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D61448

--HG--
extra : moz-landing-system : lando

dom/base/Selection.cpp

@@ -1893,7 +1893,10 @@ void Selection::AddRangeAndSelectFramesAndNotifyListeners(nsRange& aRange,
     }
   }
 
-  MOZ_ASSERT(rangeIndex >= 0);
+  if (rangeIndex < 0) {
+    return;
+  }
+
   MOZ_ASSERT(rangeIndex < static_cast<int32_t>(mRanges.Length()));
 
   SetAnchorFocusRange(rangeIndex);

dom/base/Selection.h

@@ -171,7 +171,8 @@ class Selection final : public nsSupportsWeakReference,
    *
    * @param aOutIndex points to the range last added, if at least one was added.
    *                  If aRange is already contained, it points to the range
-   *                  containing it.
+   *                  containing it. -1 if mRanges was empty and no range was
+   *                  added.
    */
   nsresult AddRangesForSelectableNodes(nsRange* aRange, int32_t* aOutIndex,
                                        bool aNoStartSelect = false);