fix jr * not setting mem_ptr

[rofl0r]

this was discovered running the core side-by-side with the cycle-stepped emulator of floooh (which is a lot slower). according to a comment here (line 80): https://github.com/anotherlin/z80emu/blob/1c418fa0d719abab9273131113defbe276101d95/README.txt zexall doesn't exercise the jr instruction at all.