Return NULL for mmap fail case on Unix

[wscho77]

If mmap failed, "MAP_FAILED" is returned not "NULL".
The windows implememtation of GetRWMapping returns "NULL" for fail case, and the caller function is also checking "NULL".
So, change Unix implementation to return "NULL" for fail case.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[Wraith2]

This is the second use of nmap that needed changing recently,
#77952 is it worth auditing all other uses?

[wscho77]

I checked all mmap return value and add missing one.

[janvorli]

LGTM, thank you! Sigh, I wonder how I could get it wrong at that many places.

[janvorli]

Ah, I've approved it accidentally, we will need to fix the thing I've commented on in around the memset.

[janvorli]

Hmm, some CI legs are failing with the newly added fatal error.
Ah, I can see the problem, the condition added in the VMToOSInterface::ReleaseDoubleMappedMemory is reverse.

[wscho77]

Hmm, some CI legs are failing with the newly added fatal error. Ah, I can see the problem, the condition added in the VMToOSInterface::ReleaseDoubleMappedMemory is reverse.

Oops. I fixed condition check. Thank you.

[janvorli]

LGTM, thank you!