input: make <Ctrl+Shift+Arrow> work also when using --rebindkeypad

The function get_keycode() should never return zero -- it should
return either the code from ncurses or the given fallback code.

This fixes https://savannah.gnu.org/bugs/?52682.

src/nano.c

@@ -1532,7 +1532,7 @@ int get_keycode(const char *keyname, const int standard)
 #ifdef HAVE_KEY_DEFINED
     const char *keyvalue = tigetstr(keyname);
 
-    if (keyvalue != 0 && keyvalue != (char *)-1)
+    if (keyvalue != 0 && keyvalue != (char *)-1 && key_defined(keyvalue))
 	return key_defined(keyvalue);
     else
 #endif