fota_tar: Improve a bit gTarFd handling

fota_tar: Improve a bit gTarFd handling

gTarFd is global fd for tar file, as managed by tar_open() & tar_close().

This commit fixes case when returned fd equals 0 (ie. program working with closed stdin)
and avoids depending on local, overwritten vars for cleanup functions, ie:

  if (gTarFd > 0)
     fd = gTarFd
  if (gTarFd < 0)
     fd = open(..)
  ...
cleanup:
  if (fd > 0)
     close(fd) // what fd it closes? gTarFd? locally opened one?

Change-Id: I83b227709ad5a97d0b0e3062f3f060cd052d98b4