[scudo][standalone] Lists fix

[cryptoad]

Summary

Apparently during the review of D69265, and my flailing around with
git, a somewhat important line disappeared.

On top of that, there was no test exercising that code path, and
while writing the follow up patch I intended to write, some CHECKs
were failing.

Re-add the missing line, and add a test that fails without said line.

[cryptoad]

Expanding the new test a bit.

compiler-rt/lib/scudo/standalone/list.h

template <class T> struct DoublyLinkedList : IntrusiveList<T> {
   using IntrusiveList<T>::empty;
 
   void push_front(T *X) {
     X->Prev = nullptr;
     if (empty()) {
       Last = X;
     } else {
       DCHECK_EQ(First->Prev, nullptr);
+      First->Prev = X;
     }
     X->Next = First;
     First = X;
     Size++;
   }
 
   // Inserts X before Y.
   void insert(T *X, T *Y) {

compiler-rt/lib/scudo/standalone/tests/list_test.cpp

TEST(ScudoListTest, DoublyLinkedList) {
   L.checkConsistency();
   L.remove(Z);
   EXPECT_EQ(L.size(), 1U);
   EXPECT_EQ(L.front(), X);
   EXPECT_EQ(L.back(), X);
   L.checkConsistency();
   L.pop_front();
   EXPECT_TRUE(L.empty());
+
+  L.push_back(X);
+  L.insert(Y, X);
+  EXPECT_EQ(L.size(), 2U);
+  EXPECT_EQ(L.front(), Y);
+  EXPECT_EQ(L.back(), X);
+  L.checkConsistency();
+  L.remove(Y);
+  EXPECT_EQ(L.size(), 1U);
+  EXPECT_EQ(L.front(), X);
+  EXPECT_EQ(L.back(), X);
+  L.checkConsistency();
+  L.pop_front();
+  EXPECT_TRUE(L.empty());
 }