Better fix for TakeWhileBetween fuzz failure

parser.go

@@ -288,21 +288,18 @@ func TakeWhileBetween(lower, upper int, predicate func(r rune) bool) Parser[stri
 			return "", "", errors.New("TakeWhileBetween: predicate never returned true")
 		}
 
-		index := -1 // Index of last char for which predicate returns true
+		index := 0 // Index of last char for which predicate returns true
 		for pos, char := range input {
 			if !predicate(char) {
 				break
 			}
 			// Add the byte width of the char in question because the next char is the
 			// first one where predicate(char) == false, that's where we want to cut
 			// the string
-			index = pos + utf8.RuneLen(char)
-		}
-
-		// This is *extremely* rare and I only found it with fuzz testing! I'm still not entirely
-		// sure why this can happen, but this is a best stab at explaining the error
-		if index == -1 {
-			return "", "", errors.New("TakeWhileBetween: invalid utf-8 byte split")
+			runeLen := utf8.RuneLen(char)
+			if runeLen != -1 {
+				index = pos + utf8.RuneLen(char)
+			}
 		}
 
 		// If we have an index, our job now is to return whichever is longest out of

parser_test.go

@@ -687,15 +687,15 @@ func TestTakeWhileBetween(t *testing.T) {
 			err:       "",
 		},
 		{
-			name:      "fuzz failure",
+			name:      "fuzz failure", // Now correctly handled!
 			input:     "\U0001925e0",
 			lower:     9,
 			upper:     83,
 			predicate: unicode.IsGraphic,
 			value:     "",
 			remainder: "",
 			wantErr:   true,
-			err:       "TakeWhileBetween: invalid utf-8 byte split",
+			err:       "TakeWhileBetween: predicate matched only 0 chars (), below lower limit (9)",
 		},
 		{
 			name:      "nil predicate",