https://docs.codewars.com/training/troubleshooting#c-error-undefinedbehaviorsanitizer-segv-on-unknown-address-0x000000000000

A shot in the dark: maybe you try to access a pointer which points to 0?

yes

I changed tests a bit so now they should give better feedback why your test failed. Now, running your solution, I get following message:

Incorrect answer for dice={3, 3, 3, 3, 6}
  Expected: equal to 300
  Actual: 0

Try this test case: Assert::That(sum_strings("111111111111111111111111111111", "111111111111111111111111111111"), Equals("222222222222222222222222222222"));