Don't exit on eof in header reading in stdcopy

There was random lines drops in docker logs because of this
Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
Merge pull request #6320 from LK4D4/fix_stdcopy_eof

Don't exit on eof in header reading in stdcopy