Fix SignCheck EndOfStreamException on RPM symlink entries

[ViktorHofer]

Problem

\RpmVerifier.ReadArchiveEntries\ yielded all CPIO entries from RPM payloads—including symlinks. In CPIO format, symlink entries contain just the target path string as data (e.g. ../../shared/foo.dll, ~30 bytes), not actual file content. These got extracted to disk as tiny text files that then failed PE header parsing with \EndOfStreamException, producing thousands of noisy errors like:

\
Outcome="Unsigned" Error="Verification error: System.IO.EndOfStreamException: Unable to read beyond the end of the stream.
at System.IO.BinaryReader.ReadUInt32()
at Microsoft.SignCheck.Interop.PortableExecutable.IMAGE_DOS_HEADER.Read(String path)
...
\\

The .NET SDK RPMs contain thousands of symlinked .dll\ files, explaining the massive error volume.

Fix

Root cause (\RpmVerifier.cs):

• Filter CPIO entries to regular files only using the entry's mode field (\CpioEntry.FileKindMask\ / \CpioEntry.RegularFile). This skips symlinks, directories, and other non-file entry types.
• Removed an unused \File.Create\ call that was creating a stray temp file.

Defense-in-depth (\ImageDosHeader.cs):

• Validate file size >= 64 bytes in \IMAGE_DOS_HEADER.Read\ before attempting to read, producing a clear \InvalidDataException\ instead of the confusing \EndOfStreamException\ from \BinaryReader.