message: Explicitly initialize prevector _union
  backports bitcoin@1d9aa008d6e043c29c3c5b030a6d04278aea233b
diff: @@ -220,13 +220,13 @@ public:
  }
  }
  - prevector() : _size(0) {}
  + prevector() : _size(0), _union{{}} {}
  - explicit prevector(size_type n) : _size(0) {
  + explicit prevector(size_type n) : prevector() {
  resize(n);
  }
  - explicit prevector(size_type n, const T& val = T()) : _size(0) {
  + explicit prevector(size_type n, const T& val = T()) : prevector() {
  change_capacity(n);
  while (size() < n) {
  _size++;
  @@ -235,7 +235,7 @@ public:
  }
  template<typename InputIterator>
  - prevector(InputIterator first, InputIterator last) : _size(0) {
  + prevector(InputIterator first, InputIterator last) : prevector() {
  size_type n = last - first;
  change_capacity(n);
  while (first != last) {
  @@ -245,7 +245,7 @@ public:
  }
  }
  - prevector(const prevector<N, T, Size, Diff>& other) : _size(0) {
  + prevector(const prevector<N, T, Size, Diff>& other) : prevector() {
  change_capacity(other.size());
  const_iterator it = other.begin();
  while (it != other.end()) {
  @@ -255,7 +255,7 @@ public:
  }
  }
  - prevector(prevector<N, T, Size, Diff>&& other) : _size(0) {
  + prevector(prevector<N, T, Size, Diff>&& other) : prevector() {
  swap(other);
  }
  