Fix UAF in aaaa on arm/thumb switching ##crash

* Reported by @peacock-doris via huntr.dev
* Reproducer tests_65185
* This is a logic fix, but not the fully safe as changes in the code
  can result on UAF again, to properly protect r2 from crashing we
  need to break the ABI and add refcounting to RRegItem, which can't
  happen in 5.6.x because of abi-compat rules