ssh/key-func.c (guile_ssh_string_to_public_key): Fix a bug

* ssh/key-func.c (guile_ssh_string_to_public_key): Fix a bug: Call
  `scm_dynwind_end' at the end of the procedure so call to the procedure
  won't lead to segfaults.
* tests/key.scm ("public-key->string"): Improve.
  ("string->public-key"): New TC.