Fix (lib/gadgets/x86) : cache_queue's fp_cache + sz_ret crash bugs

cache_queue's fp_cache crash:
- It may cause a crash as it wrote a fp_cache entry to a bad cache_queue
  pointer

sz_ret crash:
- Improper use of x86_disasm() as it would disassemble at the wrong place
  with the wrong length. If sz_ret was used for allocating memory or
  other operations, it may have caused subsequent bugs if the process
  didn't crash here.