Explicitly initialize prevector _union #14028

 174 | +            indirect.indirect = static_cast<char*>(malloc(((size_t)sizeof(T)) * _size));
 175 | +            assert(indirect.indirect);
 176 | +            indirect.capacity = _size;
 177 | +            _size += N + 1;
 178 | +            return indirect;
 179 | +        }

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 263 | @@ -248,32 +264,24 @@ class prevector {
 264 |  
 265 |      prevector() : _size(0), _union{{}} {}
 266 |  
 267 | -    explicit prevector(size_type n) : _size(0) {
 268 | -        resize(n);
 269 | +    explicit prevector(size_type n) : _size(n), _union{init_union()} {

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