Auto merge of servo#199 - upsuper:nth-boundary, r=emilio

Avoid boundary check in parse_n_dash_digits

Fix [bug 1404678](https://bugzilla.mozilla.org/show_bug.cgi?id=1404678).

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Author: bors-servo

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cssparser"
-version = "0.21.2"
+version = "0.21.3"
 authors = [ "Simon Sapin <simon.sapin@exyr.org>" ]
 
 description = "Rust implementation of CSS Syntax Level 3"

src/css-parsing-tests/An+B.json

@@ -151,6 +151,8 @@
 " +n - 1", [1, -1],
 " -n - 1", [-1, -1],
 "+ N - 1", null,
-"- N - 1", null
+"- N - 1", null,
+
+"2n\u22121", null
 
 ]

src/nth.rs

@@ -89,9 +89,10 @@ fn parse_signless_b<'i, 't>(input: &mut Parser<'i, 't>, a: i32, b_sign: i32) ->
 }
 
 fn parse_n_dash_digits(string: &str) -> Result<i32, ()> {
-    if string.len() >= 3
-    && string[..2].eq_ignore_ascii_case("n-")
-    && string[2..].chars().all(|c| matches!(c, '0'...'9'))
+    let bytes = string.as_bytes();
+    if bytes.len() >= 3
+    && bytes[..2].eq_ignore_ascii_case(b"n-")
+    && bytes[2..].iter().all(|&c| matches!(c, b'0'...b'9'))
     {
         Ok(parse_number_saturate(&string[1..]).unwrap())  // Include the minus sign
     } else {