flag: fix panic on single `-`

vlib/flag/flag.v

@@ -176,7 +176,7 @@ fn (mut fs FlagParser) parse_bool_value(longhand string, shorthand byte) ?string
 			fs.args.delete(i)
 			return val
 		}
-		if arg[0] == `-` && arg[1] != `-` && arg.index_byte(shorthand) != -1 {
+		if arg.len > 1 && arg[0] == `-` && arg[1] != `-` && arg.index_byte(shorthand) != -1 {
 			// -abc is equivalent to -a -b -c
 			return 'true'
 		}

vlib/flag/flag_test.v

@@ -344,3 +344,11 @@ fn test_long_options_that_start_with_the_same_letter_as_another_short_option_bot
 	assert verbose == true
 	assert vabc == '/abc'
 }
+
+fn test_single_dash() {
+	mut fp := flag.new_flag_parser([
+		'-'
+	])
+	flag_update := fp.bool('update', `u`, false, 'Update tools')
+	assert flag_update == false
+}