search: use a better value to mean "nothing was typed"

When the keyboard buffer is empty, return a value that is not equivalent
to <Ctrl+Space>, so that typing a bunch of these while searching is going
on will not result in some of them getting executed afterward.

src/search.c

@@ -219,7 +219,7 @@ int findnextstr(const char *needle, bool whole_word_only, int modus,
 			lastkbcheck = time(NULL);
 
 			/* Consume all waiting keystrokes until a Cancel. */
-			while (input) {
+			while (input != ERR) {
 				if (func_from_key(&input) == do_cancel) {
 					statusbar(_("Cancelled"));
 					enable_waiting();

src/winio.c

@@ -363,7 +363,7 @@ int parse_kbinput(WINDOW *win)
 	kbinput = get_input(win, 1);
 
 	if (kbinput == NULL && !waiting_mode)
-		return 0;
+		return ERR;
 
 	while (kbinput == NULL)
 		kbinput = get_input(win, 1);