degesch: fix prompt not showing up after change

When a backlog helper was running and the prompt changed,
it failed to restore within input_rl_show().

Since before input_rl_show() is called the prompt is empty
and in input_rl__restore() it will be changed to the new
version, just skip invoking any Readline functions within
input_rl_set_prompt() when the prompt is hidden.  Simple
and straight-forward.

This bug is what I hinted at in the previous commit.

degesch.c

@@ -321,17 +321,15 @@ input_rl_set_prompt (void *input, char *prompt)
 	struct input_rl *self = input;
 	cstr_set (&self->prompt, prompt);
 
-	if (!self->active)
+	if (!self->active || self->prompt_shown <= 0)
 		return;
 
 	// First reset the prompt to work around a bug in readline
 	rl_set_prompt ("");
-	if (self->prompt_shown > 0)
-		rl_redisplay ();
+	rl_redisplay ();
 
 	rl_set_prompt (self->prompt);
-	if (self->prompt_shown > 0)
-		rl_redisplay ();
+	rl_redisplay ();
 }
 
 static void