fix: ansi_c::undo() won't panic anymore.

Author: Byron

gix-quote/src/ansi_c.rs

@@ -51,7 +51,9 @@ pub fn undo(input: &BStr) -> Result<(Cow<'_, BStr>, usize), undo::Error> {
             .get(position + 1..)
             .ok_or_else(|| undo::Error::new("Unexpected end of input", input))?
             .as_bstr();
-        let next = input[0];
+        let next = *input
+            .first()
+            .ok_or_else(|| undo::Error::new("Unexpected end of input", input))?;
         *input = input.get(1..).unwrap_or_default().as_bstr();
         Ok(next)
     }

gix-quote/tests/quote.rs

@@ -80,5 +80,12 @@ mod ansi_c {
             assert_eq!(unquoted, std::borrow::Cow::Borrowed(b"hello there".as_bstr()));
             assert_eq!(&input[consumed..], " out of quote");
         }
+
+        #[test]
+        fn fuzzed() {
+            for invalid in ["\"\\", "\"Q\u{2}QT\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\0\0\\"] {
+                assert!(ansi_c::undo(invalid.into()).is_err());
+            }
+        }
     }
 }