Comment by SAI_Peregrinus

musl libc violates the POSIX documented behavior of `crypt()`¹

> Upon successful completion, crypt() shall return a pointer to the hashed password; the first two bytes of the returned value shall be those of the salt argument. Otherwise, it shall return a null pointer and set errno to indicate the error.

Musl returns '!' or '*', not null.

¹https://pubs.opengroup.org/onlinepubs/9799919799/