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.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/mixed_arena.h b/src/mixed_arena.h
index 9ae19f94d..b4822680b 100644
--- a/src/mixed_arena.h
+++ b/src/mixed_arena.h
@@ -370,7 +370,7 @@ public:
   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;