Fix saturating overflow in ExtraData parsing

- Fix overflow in ExtraData by casting byte_len to usize before
multiplication. This was previously multiplying a u16 with
saturating_mul and would overflow when the extra data in string
tables reaches a certain size.
- Add test case for parsing large string table to verify correct
stream position after read.

test_data/string_tables/ServerMapCycleLarge_meta.json

@@ -0,0 +1,4 @@
+{
+  "name": "ServerMapCycle",
+  "marker": 4039822362
+}