Make RC4 string-to-key more robust

krb5int_utf8cs_to_ucs2les() can read slightly beyond the end of the
input buffer if the buffer ends with an invalid UTF-8 sequence.  When
computing the RC4 string-to-key result, make a zero-terminated copy of
the input string and use krb5int_utf8s_to_ucs2les() instead.

(cherry picked from commit b881474)

ticket: 8576
version_fixed: 1.14.6

src/lib/crypto/krb/s2k_rc4.c

@@ -10,6 +10,7 @@ krb5int_arcfour_string_to_key(const struct krb5_keytypes *ktp,
     krb5_error_code err = 0;
     krb5_crypto_iov iov;
     krb5_data hash_out;
+    char *utf8;
     unsigned char *copystr;
     size_t copystrlen;
 
@@ -20,8 +21,11 @@ krb5int_arcfour_string_to_key(const struct krb5_keytypes *ktp,
         return (KRB5_BAD_MSIZE);
 
     /* We ignore salt per the Microsoft spec. */
-    err = krb5int_utf8cs_to_ucs2les(string->data, string->length, &copystr,
-                                    &copystrlen);
+    utf8 = k5memdup0(string->data, string->length, &err);
+    if (utf8 == NULL)
+        return err;
+    err = krb5int_utf8s_to_ucs2les(utf8, &copystr, &copystrlen);
+    free(utf8);
     if (err)
         return err;