ssh.c: add a missing delete_callbacks_for_context.

If an Ssh structure is destroyed while its IdempotentCallback
ssh->ic_out_raw is prnding, then the latter will stay on callback.c's
list pointing at the freed memory.

ssh.c

@@ -900,6 +900,8 @@ static void ssh_free(Backend *be)
 	ssh_gss_cleanup(ssh->gss_state.libs);
 #endif
 
+    delete_callbacks_for_context(ssh); /* likely to catch ic_out_raw */
+
     need_random_unref = ssh->need_random_unref;
     sfree(ssh);