Fix tokenizer crash

This would crash from the highlighter for something like

`PATH={$PATH[echo " "`

The underlying cause is that we use "char_at" which panics on
overread.

So instead this implements try_char_at and then just returns None.

fish-rust/src/tokenizer.rs

@@ -769,16 +769,9 @@ pub fn quote_end(s: &wstr, mut pos: usize, quote: char) -> Option<usize> {
     loop {
         pos += 1;
 
-        if pos == s.len() {
-            return None;
-        }
-
-        let c = s.char_at(pos);
+        let c = s.try_char_at(pos)?;
         if c == '\\' {
             pos += 1;
-            if pos == s.len() {
-                return None;
-            }
         } else if c == quote ||
                 // Command substitutions also end a double quoted string.  This is how we
                 // support command substitutions inside double quotes.

fish-rust/src/wchar_ext.rs

@@ -222,6 +222,18 @@ fn char_at(&self, index: usize) -> char {
         }
     }
 
+    /// Return the char at an index.
+    /// If the index is equal to the length, return '\0'.
+    /// If the index exceeds the length, return None.
+    fn try_char_at(&self, index: usize) -> Option<char> {
+        let chars = self.as_char_slice();
+        match index {
+            _ if index == chars.len() => Some('\0'),
+            _ if index > chars.len() => None,
+            _ => Some(chars[index]),
+        }
+    }
+
     /// \return an iterator over substrings, split by a given char.
     /// The split char is not included in the substrings.
     fn split(&self, c: char) -> WStrCharSplitIter {

tests/checks/indent.fish

@@ -432,3 +432,6 @@ echo 'begin
 echo 'multiline-\\
 -word' | $fish_indent --check
 echo $status #CHECK: 0
+
+echo 'PATH={$PATH[echo " "' | $fish_indent --ansi
+# CHECK: PATH={$PATH[echo " "