string: fix fields method when no whitespace (#9326)

vlib/builtin/string.v

@@ -1618,26 +1618,28 @@ pub fn (s string) repeat(count int) string {
 pub fn (s string) fields() []string {
 	mut res := []string{}
 	mut word_start := 0
-	mut word_end := 0
+	mut word_len := 0
 	mut is_in_word := false
 	mut is_space := false
 	for i, c in s {
 		is_space = c in [` `, `\t`, `\n`]
+		if !is_space {
+			word_len++
+		}
 		if !is_in_word && !is_space {
 			word_start = i
 			is_in_word = true
 			continue
 		}
 		if is_space && is_in_word {
-			word_end = i
-			res << s[word_start..word_end]
+			res << s[word_start..word_start + word_len]
 			is_in_word = false
-			word_end = 0
+			word_len = 0
 			word_start = 0
 			continue
 		}
 	}
-	if is_in_word && word_start > 0 {
+	if is_in_word && word_len > 0 {
 		// collect the remainder word at the end
 		res << s[word_start..s.len]
 	}

vlib/builtin/string_test.v

@@ -915,10 +915,11 @@ fn test_sorter() {
 	assert arr[2].i == 102
 }
 
-fn test_split_by_whitespace() {
+fn test_fields() {
 	assert 'a bcde'.fields() == ['a', 'bcde']
 	assert '  sss \t  ssss '.fields() == ['sss', 'ssss']
 	assert '\n xyz \t abc   def'.fields() == ['xyz', 'abc', 'def']
+	assert 'hello'.fields() == ['hello']
 	assert ''.fields() == []
 }