bugfix(map): Guard the fallback map display name against a missing separator

[bobtista]

• Closes Crash scanning a map with no map name on non-Windows builds #3100
• Depends on refactor(lib): Add path separator helpers to PathUtil #3105, whose commit is included here until it merges

Both fallback display name sites do fname.reverseFind('\\') + 1. Where '\\' is not the separator that returns null, and nullptr + 1 reaches AsciiString::set:

fname:   /home/user/GeneralsData/Maps/MyMap/MyMap.map
before:  AsciiString::set(nullptr + 1) -> crash
after:   MyMap

Now both sites call a getPathLeaf helper that accepts either separator and returns the whole name when neither is present.

Todo:

• Scan a user maps folder in which 384 maps take the fallback name path, without crashing
• Replicate to Generals — N/A, it's in Core