evil_string: fix strndup() for non-NULL terminated strings.

If the given string is not null-terminated, then strlen() will go out
of boundaries, we must limit the lookup to given 'n' parameter.

To do so use strnlen(), that is a strlen() bounded by a maximum size.

src/lib/evil/evil_string.c

@@ -18,10 +18,9 @@
 char *
 strndup(const char *str, size_t n)
 {
-   size_t slen = strlen(str);
+   size_t slen = strnlen(str, n);
    char *ret;
 
-   if (slen > n) slen = n;
    ret = malloc (slen + 1);
    if (!ret) return NULL;