[unicode-range] Avoid integer overflow (panic in debug builds)

If a long string of hex digits are passed to unicode-range, the code tries to
parse them into a number before checking whether there are more than the 6 digits
allowed by the syntax, and this may lead to integer overflow.

To avoid this, check the number of digits and error out earlier if there are
too many to possibly be valid.

(See https://bugzilla.mozilla.org/show_bug.cgi?id=1900403)

src/css-parsing-tests/urange.json

@@ -84,6 +84,11 @@
     null,
     null,
     null
+],
+
+"U+26F9200D2640, U+10000-26F9200D2640", [
+    null,
+    null
 ]
 
 ]

src/unicode_range.rs

@@ -104,7 +104,7 @@ fn parse_concatenated(text: &[u8]) -> Result<UnicodeRange, ()> {
         Some((&b'+', text)) => text,
         _ => return Err(()),
     };
-    let (first_hex_value, hex_digit_count) = consume_hex(&mut text);
+    let (first_hex_value, hex_digit_count) = consume_hex(&mut text, 6)?;
     let question_marks = consume_question_marks(&mut text);
     let consumed = hex_digit_count + question_marks;
     if consumed == 0 || consumed > 6 {
@@ -124,7 +124,7 @@ fn parse_concatenated(text: &[u8]) -> Result<UnicodeRange, ()> {
             end: first_hex_value,
         });
     } else if let Some((&b'-', mut text)) = text.split_first() {
-        let (second_hex_value, hex_digit_count) = consume_hex(&mut text);
+        let (second_hex_value, hex_digit_count) = consume_hex(&mut text, 6)?;
         if hex_digit_count > 0 && hex_digit_count <= 6 && text.is_empty() {
             return Ok(UnicodeRange {
                 start: first_hex_value,
@@ -135,19 +135,23 @@ fn parse_concatenated(text: &[u8]) -> Result<UnicodeRange, ()> {
     Err(())
 }
 
-fn consume_hex(text: &mut &[u8]) -> (u32, usize) {
+// Consume hex digits, but return an error if more than digit_limit are found.
+fn consume_hex(text: &mut &[u8], digit_limit: usize) -> Result<(u32, usize), ()> {
     let mut value = 0;
     let mut digits = 0;
     while let Some((&byte, rest)) = text.split_first() {
         if let Some(digit_value) = (byte as char).to_digit(16) {
+            if digits == digit_limit {
+                return Err(());
+            }
             value = value * 0x10 + digit_value;
             digits += 1;
-            *text = rest
+            *text = rest;
         } else {
             break;
         }
     }
-    (value, digits)
+    Ok((value, digits))
 }
 
 fn consume_question_marks(text: &mut &[u8]) -> usize {