cbreak_mode(): take no action on invalid file descriptor

src/lib/input.c

@@ -20,6 +20,9 @@ void sigwinch_handler(int signo){
 }
 
 int cbreak_mode(int ttyfd, const struct termios* tpreserved){
+  if(n->ttyfd < 0){
+    return 0;
+  }
   // assume it's not a true terminal (e.g. we might be redirected to a file)
   struct termios modtermios;
   memcpy(&modtermios, tpreserved, sizeof(modtermios));