src/base64.c (bin_to_base64): Fix an infinite loop.

* src/base64.c (bin_to_base64): Change type of the second argument
  to int to prevent an infinite loop due to wrapping of the size_t
  value.
* src/base64.h: Update.
* src/key-func.c (guile_ssh_public_key_to_string): Change type of
  `key_len'.
* NEWS: Update.