Fix an index error in ArenaVectorBase (#3486)

Because `resize()` sets `usedElements` to its argument, we were
accessing `data[usedElements]`, which can be outside of allocated memory
depending the internal state, i.e., `allocatedElements`'s value.

It is hard to come up with a test case for this because apparently the
failure condition depends on the vector's internal state.

Author: aheejin

src/mixed_arena.h

@@ -370,7 +370,7 @@ template<typename SubType, typename T> class ArenaVectorBase {
   void insertAt(size_t index, T item) {
     assert(index <= usedElements); // appending is ok
     resize(usedElements + 1);
-    for (auto i = usedElements; i > index; --i) {
+    for (auto i = usedElements - 1; i > index; --i) {
       data[i] = data[i - 1];
     }
     data[index] = item;