session-func.c: Add default case for switches.

* src/session-func.c (guile_ssh_connect)
  (guile_ssh_blocking_flush): Return 'error by default.
channel-func.c, auth.c: Fix compiler warnings.

* src/channel-func.c: Include error.h
  (guile_ssh_channel_read): Initialize obtained_data with
  SCM_BOOL_F
* src/auth.c: Include error.h
  (guile_ssh_userauth_pubkey): Initialize strings with null.