pref: fix access in invalid position on `v search ''` (#13993)

vlib/v/pref/pref.v

@@ -634,7 +634,7 @@ pub fn parse_args_and_show_errors(known_external_commands []string, args []strin
 					eprintln('Use `v $arg` instead.')
 					exit(1)
 				}
-				if arg[0] == `-` {
+				if arg.len != 0 && arg[0] == `-` {
 					if arg[1..] in pref.list_of_flags_with_param {
 						// skip parameter
 						i++

vlib/v/pref/pref_test.v

@@ -0,0 +1,8 @@
+module pref
+
+fn test_check_parametes() {
+	// reproducing issue https://github.com/vlang/v/issues/13983
+	_, cmd := parse_args_and_show_errors(['help'], [''], true)
+	// no command found from args
+	assert cmd == ''
+}