Fix crashes due to malicious RPFs

[alexguirre]

RpfFile:

• Limit RPF entry names to 256 characters because long names can cause the RPFExplorer to freeze when opening its directory.
• Skip RPFs with paths longer that 5000 characters, which are probably an attempt to make CW run out-of-memory.

ExploreForm:

• Use Path.GetExtension directly instead of FileInfo to prevent PathTooLongExceptions.
• Check for invalid characters in file names to prevent ArgumentExceptions.