better-sqlite3: Unofficial fork: help with segmentation fault (core dumped)

This just randomly came up while I was working on tests for #367 (cc @missinglink). All I did was make slight changes to the missinglink:unsafe_mode branch. I haven’t dug deeper but I assume a segfault is always bad and my changes kind of did a fuzzing? Now the question is: did I unravel an existing bug or can unsafe actually cause a segfault? Which would be a heckin bold UNSAFE if I’ve ever seen one 😄

Steps to reproduce:

1. git clone --single-branch --branch segfault git@github.com:Prinzhorn/better-sqlite3.git
2. cd better-sqlite3
3. npm install
4. npm test

  integrity checks
    Database#prepare()
      1) while iterating (allowed)
      2) while busy (blocked)
      3) while closed (blocked)
    Database#exec()
      4) while iterating (blocked)
      5) while busy (blocked)
      6) while closed (blocked)
    Database#pragma()
      7) while iterating (blocked)
      8) while busy (blocked)
      9) while closed (blocked)
    Database#checkpoint()
      10) while iterating (blocked)
      11) while busy (blocked)
      12) while closed (blocked)
    Database#backup()
      13) while iterating (allowed)
(node:13917) UnhandledPromiseRejectionWarning: TypeError: The database connection is not open
    at Database.backup (/home/alex/Documents/Projects/Bandsalat/forks/better-sqlite3/lib/backup.js:31:33)
(node:13917) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 15)
(node:13917) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
      14) while busy (allowed)
(node:13917) UnhandledPromiseRejectionWarning: TypeError: The database connection is not open
    at Database.backup (/home/alex/Documents/Projects/Bandsalat/forks/better-sqlite3/lib/backup.js:31:33)
(node:13917) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 17)
      15) while closed (blocked)
(node:13917) UnhandledPromiseRejectionWarning: TypeError: The database connection is not open
    at Database.backup (/home/alex/Documents/Projects/Bandsalat/forks/better-sqlite3/lib/backup.js:31:33)
(node:13917) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 19)
    Database#function()
      16) while iterating (blocked)
      17) while busy (blocked)
      18) while closed (blocked)
    Database#aggregate()
      19) while iterating (blocked)
      20) while busy (blocked)
      21) while closed (blocked)
    Database#loadExtension()
      22) while iterating (blocked)
      23) while busy (blocked)
      24) while closed (blocked)
    Database#close()
Segmentation fault (core dumped)
npm ERR! Test failed.  See above for more details.