fix tree-parse issue found by fuzzer (#1185)

Author: Byron

gix-object/src/tree/ref_iter.rs

@@ -104,6 +104,9 @@ fn mode_from_decimal(i: &[u8]) -> Option<(u32, &[u8])> {
         mode = (mode << 3) + (b - b'0') as u32;
         spacer_pos += 1;
     }
+    if i.len() < spacer_pos {
+        return None;
+    }
     let (_, i) = i.split_at(spacer_pos);
     Some((mode, i))
 }

gix-object/tests/tree/mod.rs

@@ -124,6 +124,11 @@ mod from_bytes {
         );
     }
 
+    #[test]
+    fn fuzzed() {
+        assert!(gix_object::TreeRef::from_bytes(b"2").is_err(), "fail, but don't crash");
+    }
+
     #[test]
     fn special_trees() -> crate::Result {
         for (name, expected_entry_count) in [