[vfs] [posix] SMBFile: fix Read() with zero size and null buffer
[vfs] [posix] SMBFile: fix reading files opened with READ_TRUNCATED flag
[vfs] [posix] SMBFile::GetPosition() return correct values on errors
[vfs] [posix] SMBFile::GetLength() return correct values on errors
[vfs] [posix] SMBFile::Write() remove unneeded cast
Merge pull request #5660 from Karlson2k/fix_posix_smb

Fix SMB files reading on POSIX platforms