flag: fix panic when given an empty string as args (#10937)

vlib/flag/flag.v

@@ -212,7 +212,7 @@ fn (mut fs FlagParser) parse_value(longhand string, shorthand byte) []string {
 			should_skip_one = false
 			continue
 		}
-		if arg[0] != `-` {
+		if arg.len == 0 || arg[0] != `-` {
 			continue
 		}
 		if (arg.len == 2 && arg[0] == `-` && arg[1] == shorthand) || arg == full {

vlib/flag/flag_test.v

@@ -405,3 +405,8 @@ fn test_dashdash_acts_as_parser_full_stop_dashdash_at_end() ? {
 	args := fp.finalize() ?
 	assert args.len > 0
 }
+
+fn test_empty_string_with_flag() {
+	mut fp := flag.new_flag_parser([''])
+	s := fp.string('something', `s`, 'default', 'Hey parse me')
+}