Fix getpwuid_r error handling

[ghost]

from man getpwuid: "In case of error, an error number is returned, and NULL is stored in *result."
So, error returned, NOT -1 and errno.
Example from man:

    if (result == NULL) {
        if (s == 0)
            printf("Not found\n");
        else {
            errno = s;
            perror("getpwnam_r");
        }
        exit(EXIT_FAILURE);
    }