input: don't discard the first keystroke after a resize when using Slang

With ncurses, a window resize will cause getch() to return immediately
with some dummy value, which nano can discard.  But with Slang, getch()
will return only when the next keystroke is typed, so the received code
should then not be discarded.

This fixes https://savannah.gnu.org/bugs/?57507.

src/winio.c

@@ -173,6 +173,9 @@ void run_macro(void)
  * in the keystroke buffer. */
 void read_keys_from(WINDOW *win)
 {
+#if defined(USE_SLANG) && !defined(NANO_TINY)
+	bool skip_others = FALSE;
+#endif
 	int input = ERR;
 	size_t errcount = 0;
 
@@ -193,7 +196,11 @@ void read_keys_from(WINDOW *win)
 #ifndef NANO_TINY
 		if (the_window_resized) {
 			regenerate_screen();
+#ifdef USE_SLANG
+			skip_others = TRUE;
+#else
 			input = KEY_WINCH;
+#endif
 		}
 #endif
 		if (input == ERR && !waiting_mode) {
@@ -220,6 +227,10 @@ void read_keys_from(WINDOW *win)
 	/* If we got a SIGWINCH, get out as the win argument is no longer valid. */
 	if (input == KEY_WINCH)
 		return;
+#ifdef USE_SLANG
+	if (skip_others)
+		return;
+#endif
 #endif
 
 	/* Read in the remaining characters using non-blocking input. */