hw/display/artist: Refactor artist_rop8() to avoid buffer over-run

hw/display/artist: Refactor artist_rop8() to avoid buffer over-run

Invalid I/O writes can craft an offset out of the vram_buffer range.
Instead of passing an unsafe pointer to artist_rop8(), pass the vram_buffer and
the offset. We can now check if the offset is in range before accessing it.

We avoid:

  Program terminated with signal SIGSEGV, Segmentation fault.
  284             *dst &= ~plane_mask;
  (gdb) bt
  #0  0x000056367b2085c0 in artist_rop8 (s=0x56367d38b510, dst=0x7f9f972fffff <error: Cannot access memory at address 0x7f9f972fffff>, val=0 '\000') at hw/display/artist.c:284
  #1  0x000056367b209325 in draw_line (s=0x56367d38b510, x1=-20480, y1=-1, x2=0, y2=17920, update_start=true, skip_pix=-1, max_pix=-1) at hw/display/artist.c:646

Reported-by: LLVM libFuzzer
Buglink: https://bugs.launchpad.net/qemu/+bug/1880326
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Helge Deller <deller@gmx.de>