Clarify when ldisc->term may be NULL.

Namely, any ldisc that you send actual data through should have a
terminal attached, because the ldisc editing/echoing system is
designed entirely for use with a terminal. The only time you can have
an ldisc with no terminal is when it's only ever used by the backend
to report changes to the front end in edit/echo status, e.g. by Unix
Plink.

Coverity spotted an oddity in ldisc_send which after a while I decided
would never have actually caused a problem, but OTOH I agree that it
was confusing, so now hopefully it's less so.

ldisc.c

@@ -7,6 +7,7 @@
 
 #include <stdio.h>
 #include <ctype.h>
+#include <assert.h>
 
 #include "putty.h"
 #include "terminal.h"
@@ -139,6 +140,17 @@ void ldisc_send(void *handle, char *buf, int len, int interactive)
 	ldisc_update(ldisc->frontend, ECHOING, EDITING);
 	return;
     }
+
+    /*
+     * If that wasn't true, then we expect ldisc->term to be non-NULL
+     * hereafter. (The only front ends which have an ldisc but no term
+     * are those which do networking but no terminal emulation, in
+     * which case they need the above if statement to handle
+     * ldisc_updates passed from the back ends, but should never send
+     * any actual input through this function.)
+     */
+    assert(ldisc->term);
+
     /*
      * Notify the front end that something was pressed, in case
      * it's depending on finding out (e.g. keypress termination for
@@ -146,7 +158,7 @@ void ldisc_send(void *handle, char *buf, int len, int interactive)
      */
     frontend_keypress(ldisc->frontend);
 
-    if (interactive && ldisc->term) {
+    if (interactive) {
         /*
          * Interrupt a paste from the clipboard, if one was in
          * progress when the user pressed a key. This is easier than