Bug 1730534 - Part 1: Assert state of SegmentedVector iterator is valid r=mccr8

The next patches remove elements from a segmented vector that is being
iterated. This patch adds assertions to ensure that we don't attempt to use an
iterator that points to a removed element.

The assertions are added to Done() because all the other methods call that.

Differential Revision: https://phabricator.services.mozilla.com/D125426

mfbt/SegmentedVector.h

@@ -289,7 +289,11 @@ class SegmentedVector : private AllocPolicy {
     }
 
    public:
-    bool Done() const { return !mSegment; }
+    bool Done() const {
+      MOZ_ASSERT_IF(mSegment, mSegment->isInList());
+      MOZ_ASSERT_IF(mSegment, mIndex < mSegment->Length());
+      return !mSegment;
+    }
 
     T& Get() {
       MOZ_ASSERT(!Done());