segfaults on mapper 218

[zlago]

Describe the bug

writing to mirrors of CIRAM under mapper 218 likely causes buffer overruns

i do know for sure that it can causes segfaults

To Reproduce

1. download the zip file from here https://forums.nesdev.org/viewtopic.php?p=310352&sid=e8a48549fa39a2f30f4f6115b17de183#p310352
2. extract
3. run the ROM in FCEUX
4. segmentation fault (core dumped)

Expected behavior
FCEUX does not close

Screenshots
If applicable, add screenshots to help explain your problem.

• OS and Version: arch linux (last updated a month ago)
• Version of FCEUX: master branch, QT

Additional context
have a stack trace at the time of segfault, it might help

#0  0x0000555555658605 in GetCHRAddress (A=21845) at /home/sylvie/util/fceux/src/ppu.cpp:477
#1  0x0000555555661302 in FCEUX_PPU_Loop (skip=0) at /home/sylvie/util/fceux/src/ppu.cpp:2218
#2  0x000055555566095c in FCEUPPU_Loop (skip=0) at /home/sylvie/util/fceux/src/ppu.cpp:1774
#3  0x0000555555627080 in FCEUI_Emulate (pXBuf=0x7fffc77fd098, SoundBuf=0x7fffc77fd0a0, SoundBufSize=0x7fffc77fd090, skip=0) at /home/sylvie/util/fceux/src/fceu.cpp:845
#4  0x0000555555871c7e in DoFun (frameskip=0, periodic_saves=0) at /home/sylvie/util/fceux/src/drivers/Qt/fceuWrapper.cpp:1363
#5  0x0000555555872120 in fceuWrapperUpdate () at /home/sylvie/util/fceux/src/drivers/Qt/fceuWrapper.cpp:1528
#6  0x000055555573c59a in emulatorThread_t::run (this=0x555556907dd0) at /home/sylvie/util/fceux/src/drivers/Qt/ConsoleWindow.cpp:5113
#7  0x00007ffff19784fb in ??? () at /usr/lib/libQt6Core.so.6
#8  0x00007ffff0e97f42 in ??? () at /usr/lib/libc.so.6
#9  0x00007ffff0f1efac in ??? () at /usr/lib/libc.so.6