Not to mention the fact that the issue is that the data structures themselves are trashed. So even if what you're proposing "fixes" the issue, it "fixes" it by prolonging the duration of time the driver is going to be using fucked-up values and doing wrong shit. Your "fix" is, like, MASH-level bad.
nothing wrong with *safer* code in general, but no, people can't understand the basics of masking probably shouldn't even be allowed access to the MAME / MESS code. I guess it was probably just a & / % mixup tho, but using % is *very* expensive on performance so not a good solution anyway.