The function itself is correct, but in your test program, the statement that calls the function causes undefined behaviour by using the value of an uninitialized variable.
The bug is in the calling function, and it could be detected by code review or static analysis of the calling function. Using your compiler explorer link, the gcc 8.2 compiler does detect the bug. (Maybe you could file a bug report against clang that it doesn't find the problem).
Undefined behaviour means anything can happen, which includes the program crashing a few lines after the event that triggered the undefined behaviour.
NB. The answer to "Can undefined behaviour cause _____ ?" is always "Yes". That's literally the definition of undefined behaviour.
