In order to test the changes, I attempted to recreate the segfault in the most current commit to cpython master and in your remote branch bpo-34543-struct-crashes.

cpython master:

PR branch:

For the version used for testing the latest cpython, I used the function test_segfault() on the second round in order to perform the test multiple times consecutively. The issue tracker reported similar problems with replication. The first round caused a TypeError, and the second one caused the segfault.

After performing test_segfault() 3 times consecutively in the PR's branch, ValueError was raised each time with the same message. As far as I can tell, this resolves the segfault issue and provides a significant improvement by raising a consistent exception each time.

Nicely done @ZackerySpytz, approved.